Document:

<PAGE>   1
                                                                   EXHIBIT 10.21

              GREATER CAPITAL AREA ASSOCIATION OF REALTORS(R), INC.

                    SHORT FORM COMMERICAL AND APARTMENT HOUSE
                         REAL ESTATE PURCHASE AGREEMENT

                                Date of Offer:  July 27, 1999

     THIS PURCHASE AGREEMENT ("Agreement") is made and entered into by and
between United Therapeutics Corp. ("Purchaser"), and 1106 Spring Street
Associates ("Seller").

     The parties hereto are desirous of entering into an agreement relating to
the sale by Seller to Purchaser of certain property. In consideration of the
foregoing and of the mutual covenants and agreements hereinafter set forth, the
parties hereto hereby agree as follows:

     1. DEPOSIT: DESCRIPTION OF PROPERTY. Hanes Realty, Inc. ("Agent") has
received from Purchaser a deposit ("Deposit") wire transfer (by check, by cash,
by note) of Seventy-Five Thousand Dollars ($75,000.00) to be applied as part
payment of the Purchase Price of the real property legally described as Lot P4,
Square Blk "A" with improvements thereon known by street address as 1106 Spring
Street, Silver Spring, Maryland 20910 a three story and basement office/medical
building in J.C. Wilson estates in Montgomery County together with the
easements, rights, privileges and appurtenances belonging to the same, and
including all of the furniture, fixtures, furnishings, machinery, equipment and
(see addendum) (the "Personal Property") owned by Seller situated on or about
said property (said real and personal property being collectively referred to
herein as the "Property"). Seller agrees to sell and convey to Purchaser, and
Purchaser agrees to buy from Seller, the Property, upon the terms and conditions
herein set forth.

     2. PURCHASE PRICE. The purchase price ("Purchase Price") of the Property is
One Million, Five Hundred and Forty Thousand dollars ($1,540,000.00). If the
Property shall be encumbered by a mortgage or a deed of trust which Purchaser is
to assume or take title subject to and if any regular payments on account of the
principal thereof shall be made by the Seller between the date of this Agreement
and the date of settlement, the aggregate amount of any such principal payments
shall be DNA (paid to Seller by Purchaser at settlement in addition to the
amount set forth in paragraph 3(a) hereof; added to the principal amount of the
DNA Deed of Trust provided for below.)

     3. PAYMENT OF PURCHASE PRICE. [FINANCING CONTINGENCY (IF ANY) AND
ADDITIONAL OR ALTERNATIVE FINANCING TERMS (IF ANY) ARE ADDED BY ADDENDUM
     YES    XX  NO]
-----     -----

The Purchaser agrees:

     To pay One Million, Five Hundred and Forty Thousand Dollars ($1,540,000.00)
cash on the date of settlement, of which the Deposit shall be a part.

     4. TITLE. The Property is sold free of encumbrance except as aforesaid and
except for the rights of tenants in possession; title is to be good of record
and marketable, subject, however, to covenants, rights of way, easements,
conditions and restrictions of record, if any, otherwise, said Deposit is to be
returned and this Agreement declared void at the option of the Purchaser, unless
the defects are of such character that they may be readily remedied by legal
action or the Purchaser fails to object as provided below. The Seller and Agent
are hereby expressly released from all liability for damages by reason of any
defect in unmarketability of the title. In case legal steps are necessary to
perfect the title, such action must be taken by the Seller promptly at his own
expense, whereupon the time herein specified for full settlement by the
Purchaser will thereby be extended for period necessary for such perfecting of
title. Purchaser hereby authorizes Agent to order the examination of title from
_________ ("Title Company") a survey (if required), and the preparation of all
necessary conveyancing papers, at Purchaser's expense; provided, however, that
if upon examination the title should be found defective or unmarketable, and is
not

<PAGE>   2

remedied as aforesaid, the Seller hereby agrees to pay the cost of the
examination of the title and also to pay the Agent herein the commission
hereinafter provide for just as though the sale had actually been consummated
and the parties had complied with all the terms of this Agreement. All claims of
defect in or unmarketability of title by Purchaser shall be delivered by
Purchaser to Seller within fifteen (15 days) of issuance of the title report and
shall be deemed waived if not so delivered. Issuance by a title insurance
company of a title insurance commitment naming Purchaser as the proposed insured
for an owners' policy of title insurance on American Land title Association
standard form of owners' marketability policy (ALTA Form B-1970, as from time to
time amended), conforming to the requirements of this Agreement, shall be
conclusive evidence of marketability of title.

     5. DEED. Seller agrees to execute and deliver at settlement a good and
sufficient special warranty deed: or, if the Property is located in the
Commonwealth of Virginia, seller agrees to execute and deliver at settlement a
good and sufficient general warranty deed.

     6. ADJUSTMENTS. Rents, taxes, water rent, insurance and interest on
existing encumbrances (if any), cost of fuel in storage tanks, salaries and
accrued benefits of employees (if any), and operating charges (including,
without limitation, utilities charges) are to be adjusted to the date of
transfer. Rental security deposits, if any, plus any interest thereon shall be
transferred to Purchaser at the time of settlement. Taxes, general and special,
are to be adjusted according to the certificate of taxes as issued by the
Collector of Taxes, except that assessments for improvements completed prior to
the date hereof, whether assessment therefore has been levied or not, shall be
paid the Seller or allowance made therefor at the time of transfer. If the
Property is serviced by the Washington Suburban Sanitary Commission, annual
benefit charges of said Commission are to be adjusted to date of transfer and
assumed thereafter by Purchaser.

     7. CLOSING AND RECORDING COSTS. Examination of title (except as otherwise
provided above), tax certificate, conveyancing, notary fees, survey if required,
State revenue stamps, if any, and all recording charges, including those for
purchase money trust, if any, are to be at the cost of the Purchaser. Transfer
and recordation taxes shall be equally divided between Seller and Purchaser.

     8. SETTLEMENT: FORFEITURE OF DEPOSIT. On or before the 27th day of August,
1999, or as soon thereafter as (a) the report on the title can be secured if
promptly ordered by the Purchaser, (b) an appointment for settlement can be
arranged with the Title Company, (c) a survey procured, if one is necessary, and
(d) any and all contingencies herein set forth are satisfied, the Seller and
Purchaser agree to and are required to make full settlement of said purchase in
accordance with the terms hereof. At the settlement, in addition to all other
documents required to be delivered by the provisions hereof (a) Seller shall
deliver to Purchaser a bill of sale for the Personal Property, an assignment of
such insurance policies on the Property as shall be assignable, all books and
records pertaining to the operation of the Property, plans and specifications of
the Property in Seller's possession, assignments of the leases described in
paragraph 11 hereof, such certificates, permits and licenses with respect to the
Property as shall be assignable, and DNA (additional items, if any); and (b)
Purchaser shall deliver to Seller a promissory note an deed of trust, in a form
reasonably satisfactory to Seller, for financing (if any) taken back by Seller
in accordance with the terms hereof, a security agreement and financing
statement secured by the Personal Property, and DNA (additional items if any).
As of the settlement date (a) no part of the Property shall have been acquired,
or shall be about to be acquired, by any governmental authority or agency in the
exercise of its power of eminent domain or by private purchase in lieu thereof,
nor shall there by any threat or imminence of any such acquisition or purchase;
and (b) all representations and warranties as set forth in this Agreement shall
be true and correct.

        If Purchaser shall fail to make full settlement in accordance herewith,
the Deposit herein provided for may be forfeited as liquidated damages at the
option of the Seller, in which event the Purchaser shall be relieved from
further liability hereunder.

        If after breach by Purchaser, the Seller (a) elects to declare the
Purchaser's Deposit forfeited under the provisions of this Agreement, in any
such event, the Seller and Purchaser agree that Agent shall be paid by Seller,
or shall retain from the Deposit (if then in escrow), one-half (1/2) of said
Deposit as compensation for services rendered by Agent, said amount not to
exceed the full commission as hereinafter provided. If Seller elects to declare
the Deposit forfeited, the Purchaser agrees that the Agent may deliver the
remaining one-half (1/2) of said Deposit to the Seller, to be retained as
liquidated damages; and when the foregoing has been accomplished, the Purchaser
and Seller hereby expressly relieve and release said Agent from any further
liability or responsibility.

     9. PLACEMENT OF SETTLEMENT AND TITLE COMPANY. Settlement is to be made at
the Title Company or other agency employed to examine the title. Deposit with
the Title Company or such other agency of that part of the Purchase Price to be
paid in case, the deed of conveyance for execution and such other papers as are
required of either the Seller or Purchaser by the terms of this Agreement,
within the time above provided for the

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consummation of this Agreement, shall be considered good and sufficient tender
of performance by either of said parties of the terms hereof. IT IS UNDERSTOOD
THAT THE PURCHASER HAS THE RIGHT TO NAME AND EMPLOY A PARTICULAR TITLE INSURANCE
COMPANY, SETTLEMENT, OR ESCROW COMPANY OR TITLE ATTORNEY, WHICH HAS BEEN
DESIGNATED BY PURCHASER AS SET FORTH IN PARAGRAPH 4 ABOVE.

    10. DISPOSITION OF DEPOSIT. The entire Deposit, receipt of which is hereby
acknowledged, shall be held by Agent as escrow agent and deposited in an account
in accordance with applicable law until settlement hereunder is made, or until
disposition thereof is made pursuant to the terms of this Agreement. Agent shall
have the right to disburse the Deposit to Purchaser or Seller and the interest
earned thereon to Purchaser (a) at the settlement, or (b) otherwise as provided
in this Agreement upon ten (10) days' written notice to the parties, provided,
however, that Agent shall not have received any written objections to such
disbursement within ten (10) days after receipt by Purchaser and Seller of said
notice. The parties hereto hereby acknowledge that Agent shall have no liability
to any party on account of Agent's failure to disburse the Deposit or interest
thereon if a dispute shall have arisen with respect to the propriety of such
disbursement; and, in the event of any dispute as to who is entitled to receive
the Deposit or interest earned thereon, Agent shall have the right to retain the
funds and disburse them in accordance with the final order of a court of
competent jurisdiction, or to deposit such funds with said court pending a final
decision of such controversy. The parties further agree that Agent shall not be
liable for failure of any depository in which the Deposit is placed, and that
Seller and Purchaser each will indemnify, defend and save harmless Agent from
and against every claim, liability, cost, damage, loss and expense arising out
of the disbursement of the Deposit or failure to disburse the Deposit by Agent,
except in the case of Agent's gross negligence or willful misconduct.

    11. EXISTING TENANCY. The Property is sold and shall be conveyed subject to
an existing tenancy or tenancies as follows: as listed in addendum. Without the
prior written consent of Purchaser (which shall not be unreasonably withheld),
Seller shall not (a) modify the terms or terminate any such tenancies, except
for nonpayment of rent; or (b) enter into any new leases or tenancies with
respect to the property.

    12. POSSESSION. Seller agrees to give possession of the Property at time of
settlement, subject to the aforesaid existing tenancy or tenancies. If Seller
shall fail so to do and occupies said Property, Seller shall be a DNA (tenant by
sufferance, trespasser), and Seller agrees to pay Purchaser $DNA per week for
use and occupancy, and hereby expressly waives all notice to quit, if any,
provided by law.

    13. RESIDENTIAL TENANCIES. (This clause is inapplicable and of no further
effect if the Property is not subject to residential tenancy.) Within DNA days
following the Date of Agreement, Seller shall provide to any residential tenant,
subtenant, lessee, sublessee, or other person entitled to the possession,
occupancy or benefits of a rental unit in the Property or any group thereof (the
"Tenants") and to any governmental entity such notice of the intended sale of
the Property and of their rights, as shall be required by applicable law. It is
expressly agreed that the obligations of Seller and Purchaser to make settlement
hereunder are conditioned upon the right of the Tenants and/or such governmental
entity to an opportunity to purchase the Property, as and if provided by the
applicable law. The Seller hereby expressly reserves the right to sell the
Property to the Tenants and/or such governmental entity on terms which comply
with the applicable law. Upon settlement of the sale of the Property to the
Tenants, and/or such governmental entity, this Agreement shall be void and the
Deposit shall be returned to the Purchaser within 15 days of the settlement
thereunder; provided, however, that in the event of any such settlement, Seller
shall pay to Agent the commission provided for herein. The Seller shall keep
purchaser and Agent apprised of all negotiation, correspondence, contracts and
other developments with respect to the negotiations with the Tenants and/or such
governmental entity under the applicable law.

    14. RISK OF LOSS. Seller assumes the risk of loss or damage to the Property
by fire or other casualty until the executed deed of conveyance is delivered to
Purchaser or is recorded for him by the title company making the settlement.

    15. LOCAL VIOLATIONS; CONDITION AND OPERATION OF PROPERTY. All written
notices of violations of statutes, municipal order or regulations issued by any
department of the jurisdiction in which the Property is situated, or
prosecutions in any of the courts of the jurisdiction in which the Property is
situated on account thereof, against or affecting the Property that have been
received by Seller as of the Date of Agreement shall be complied with by the
Seller and the Property conveyed free thereof, Seller sates that, tot he best of
his knowledge, all such notices and prosecutions are as set forth in the Addenda
hereto. Purchaser agrees that, except as expressly provided herein, Seller has
made no representation, warranty or other statement as to the physical
condition, operation or any other matter or thing affecting the Property.
Purchaser has inspected the Property, is

<PAGE>   4

fully familiar with the condition thereof, and Purchaser agrees to take the
Property AS IS as of the Date of Agreement. Seller shall: (a) deliver the
Property in substantially the same physical condition as of the Date of
Agreement; (b) not defer normal maintenance of the Property during the period
from the Date of Agreement to the date of settlement; and (c) not enter into,
modify or terminate any maintenance or service contracts relating to the
Property prior to the date of settlement without the prior written consent of
Purchaser.

    16. CHOICE OF LAW; REFERENCES TO DISTRICT OF COLUMBIA. This Agreement, the
rights and obligations of the parties hereto, and any claims or disputes
relating thereto shall be governed by and construed in accordance with the laws
of the jurisdiction in which the Property is located (but excluding the choice
of law rules thereof) (the "applicable law"). If the Property involved in this
Agreement is located in a jurisdiction other than the District of Columbia,
wherever any reference is made to the District or any official thereof, the name
of the jurisdiction in which the Property is located and the proper official
thereof is substituted automatically.

    17. AGENT. Seller recognizes Hanes Realty, Inc. (the "Agent) as agent
negotiating this Agreement and agrees to pay to agent a commission for services
rendered amounting to 6% of the gross Purchase Price set forth herein. The
amount of commission, as aforesaid, is hereby assigned to the Agent by Seller
out of the proceeds of sale and the Deposit. The party through or by whom
settlement hereof is made is hereby irrevocably authorized and directed to make
deduction of the aforesaid commission from the proceeds of sale and to make
payment thereof to said Agent after the proper credit for the Deposit, if any,
retained by Agent, under and by virtue of the terms hereof. However, should
settlement fail to occur within the time herein set forth, Agent shall
nonetheless be entitled to receive the commission herein provided.

    18. GENERAL MASTER PLANS; SEWAGE DISPOSAL; SUBDIVISION PLAT. IF THE
PROPERTY IS LOCATED IN MONTGOMERY COUNTY, MARYLAND, EXECUTE AND APPEND ADDENDUM
ENTITLED "GENERAL MASTER PLANS; SEWAGE DISPOSAL; SUBDIVISION PLAT."

    19. D.C. SOIL DISCLOSURE REQUIREMENTS. (This clause is inapplicable and of
no further effect if Property is not located in the District of Columbia.) The
characteristics of the soil on the Property as described by the Soil
Conservation Service of the United States of Agriculture in the Soil Survey of
the District of Columbia published in 1976 and as shown on the Soil Maps of the
District of Columbia is DNA. For further information, the Purchaser can contact
a soil testing laboratory, the District of Columbia Department of Environmental
Services, or the Soil Conservation Service of the Department of Agriculture.

    20. TIME OF ESSENCE. The Purchaser and Seller agree that time is of the
essence of this Agreement.

    21. ACCEPTANCE DATE OF AGREEMENT. This Agreement must be ratified and
accepted by the Seller and spouse (if Seller is married), within five days from
the date set forth under the title hereof (Date of Offer), in order to be
effectual and binding; otherwise the obligation of the Purchaser hereunder shall
cease and terminate and the Deposit shall be refunded. The Date of this
Agreement ("Date of Agreement") shall be the date on which this Agreement is
ratified and accepted as aforesaid.

    22. BINDING EFFECT; ENTIRE AGREEMENT. Purchaser and Seller mutually agree
that this Agreement shall be binding upon them, and their respective heirs,
executors, administrators, successors and assigns; that the provisions hereof
shall survive the execution and delivery of the deed aforesaid and shall not be
merged therein; that this Agreement contains the final and entire Agreement
between the parties hereto, and that they shall not be bound by any terms,
conditions, statements, warranties, or representations, oral or written, express
or implied, not herein contained.

    23. ASSIGNMENT. This Agreement may be assigned by either party hereto
without the consent of the other party hereto, unless paragraph 3(b) or (c)
(Deeds of Trust) is applicable, in which event, this Agreement shall not be
assigned without the prior written consent of Seller.

    24. GRANTEE. The Property is to be conveyed in the names(s) to be
designated in writing by Purchaser prior to settlement.

    25. NO REPRESENTATIONS. THE AGENT ASSUMES NO RESPONSIBILITY FOR THE
CONDITION OF THE PROPERTY NOR FOR THE PERFORMANCE OF THIS AGREEMENT BY ANY OR
ALL PARTIES HERETO PURCHASER, HEREBY WARRANTS AND REPRESENTS UNTO SELLER AND THE
REAL ESTATE BROKERS HEREIN (INCLUDING AGENT) THAT NO AGENT, SERVANT OR EMPLOYEE
OR SAID REAL ESTATE BROKERS HAS MADE ANY STATEMENT, REPRESENTATION OR WARRANTY,
ORAL OR WRITTEN, EXPRESS OR IMPLIED, TO PURCHASER REGARDING THE CONDITION OF THE
PROPERTY OF ANY PART THEREOF UPON WHICH PURCHASE HAS RELIED AND WHICH IS NOT
EXPRESSLY SET FORTH IN THIS AGREEMENT.

<PAGE>   5

    26. ADDITIONAL PROVISION. Special provisions in the attached Addendum,
bearing the signatures of all parties hereto, and all plats and surveys attached
hereto, are incorporated herein and made a part hereof. Addendum attached:
 XX  yes     no; Plat attached     yes  XX  no; Survey attached:    yes
----     ----                  ----    ----                     ----
 XX  no.
----

    27. PRONOUNS. The words "Seller," "Purchaser," all pronouns and any
variations thereof shall be deemed to refer to the masculine, feminine, neuter,
singular or plural as the identity of the person or entity and the context may
require.

    28. NOTICES. All notices required or permitted herein shall be in writing
and effective as of the date on which such notice is mailed in any United States
Post Office, by certified or registered mail, postage prepaid, or hand-delivered
to Seller, the Agent or Purchaser (as shall be applicable) at the addresses
designated herein, or to such other addresses as the parties may designate in
writing from time to time.

    We, the undersigned, hereby ratify, accept and agree to the above and
acknowledge it to be our Agreement.

THIS IS A LEGALLY BINDING AGREEMENT , PURCHASER AND SELLER SHOULD SEEK COMPETENT
LEGAL ADVICE BEFORE SIGNING.

<TABLE>
<S>                                                           <C>
/s/ M. Rothblatt for                                          /s/ Judith Danish, Managing Partner
--------------------------------------------                  -------------------------------------
Purchaser                                                     Seller

United Therapeutics Corp.                                     for 1106 Spring Street Associates
--------------------------------------------                  -------------------------------------
Purchaser                                                     Seller

                                                              DATE OF RATIFICATION AND ACCEPTANCE
                                                              (DATE OF AGREEMENT)

         7/30/                      , 1999                    9:40 P.M.      8/4/ 1999
------------------------------------------                    ------------------------------------------------

Address of Purchaser:                                         Address of Seller:

United Therapeutics                                           4932 Cloister Drive
------------------------------------------                    ------------------------------------------------
1110 Spring Street                                            Rockville, MD  20852
------------------------------------------                    ------------------------------------------------
Silver Spring, MD  20910
------------------------------------------

Telephone:                                                    Telephone:
     Residence    301-254-2462                                          Residence       301-530-9559
                  ----------------                                                ------------------------
     Office       301-608-9292                                          Office          NA
                  ----------------                                                ------------------------

Agent hereby acknowledges receipt of the Deposit and agrees to hold same pursuant to the terms of this Agreement

AGENT:            Hanes Realty, Inc.
     By
                  -------------------------------
     Title        President/Broker                 (Officer must sign)
                  -------------------------------
     Address:     519 Kennedy Street N.W.
                  -------------------------------
                  Washington, DC  20011
                  -------------------------------
     Telephone:   202-882-6800
                  -------------------------------
</TABLE>

<PAGE>   6

                                    ADDENDUM

                              Additional Provisions

Addendum to contract dated July 27, 1999 between UNITED THERAPEUTICS CORP.,
Purchaser and 1106 SPRING STREET ASSOCIATES, Seller

1.    Re: personal property -

     a.       all cleaning equipment (i.e. carts, supplies, electric buffers,
              etc.) are the property of the cleaning company

     b.       all equipment, both medical and office, in the Suite 301,
              currently occupied by Dr. Louis Butler, is the property of 1106
              Spring Street Associates.

2.   All existing service contracts (i.e. trash, elevator maintenance, cleaning)
     can be assigned or cancelled at the direction of the Purchaser.

3.   The management company, Hanes Realty, Inc., will work with the Purchaser in
     the orderly transfer of all documents and information pertaining to the
     building.

4.    Existing Tenancy:

<TABLE>
<CAPTION>
Suite #           Tenant                             Lease Expires     Monthly Rent
---------------------------------------------------------------------------------------
<S>               <C>                                <C>               <C>
#1                Community Radiology                6/3/00            $ 800.00
#2                Dr. Bernadette Soong               2/28/00             749.00
#3                Dr. Mofikpara Wright               4/30/02             641.50*
#4/5              Dr. Howard Martin                  4/30/02           1,316.00
#10               Levels of Elevation                9/30/03             845.00
#20               Glick Opticians                    5/31/00           1,800.00
#101              Community Radiology                6/30/00           3,200.00
#200              Dr. Crystal McGinty                2/28/02           2,811.62
#205              Affinity Physical Ther.            12/14/01          2,123.00
#301              Dr. Louis Butler                   m/t/m             1,216.00
#303              Dr. Stuart Dulin                   4/30/02           1,316.00
#305              Dr. Jonathan Lyons                 5/31/01           3,200.00
</TABLE>

5.   The Sellers are to pay the following settlement charges:
     State Recordation Tax..................$ 6,776.00
     Loan Origination fee 2 pts.............$23,100.00
     State Transfer Tax.....................$ 7,700.00
     County Transfer Tax....................$15,400.00
                                            ----------
                                            $52,976.00

     Broker's Commission and Seller's legal fees

<TABLE>
<CAPTION>
Purchaser:                                                    Seller:

                                                              1106 Spring Street Associates
<S>                                         <C>
By:      /s/ M. Rothblatt                   By:      /s/ Judith Danish, Managing Partner
         ---------------------------                 -----------------------------------

Date:    7/30/99                            Date:    8/4/99     9:40 pm
         ---------------------------                 ------------------
</TABLE><PAGE>   1
       EXHIBIT 4.9

       THIS WARRANT AND THE SHARES OF COMMON STOCK ISSUABLE UPON EXERCISE HEREOF
       HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED
       (THE "ACT"), OR ANY STATE SECURITIES OR BLUE SKY LAWS AND MAY NOT BE
       OFFERED, SOLD, TRANSFERRED, HYPOTHECATED OR OTHERWISE ASSIGNED EXCEPT (1)
       PURSUANT TO A REGISTRATION STATEMENT WITH RESPECT TO SUCH SECURITIES
       WHICH IS EFFECTIVE UNDER THE ACT OR (2) PURSUANT TO AN AVAILABLE
       EXEMPTION FROM REGISTRATION UNDER THE ACT RELATING TO THE DISPOSITION OF
       SECURITIES AND (3) IN ACCORDANCE WITH APPLICABLE STATE SECURITIES AND
       BLUE SKY LAWS.

                             PLACEMENT AGENT WARRANT

                WARRANT TO PURCHASE 78,750 SHARES OF COMMON STOCK

                                       OF

                           SONUS COMMUNICATIONS, INC.

                       Date of Issuance: January 21, 1999

       THIS CERTIFIES that, for value received, L. Flomenhaft & Co., a New York
corporation (the "Placement Agent"), or its assigns (each a "Holder" and
collectively, the "Holders") are entitled to purchase, subject to the provisions
of this Warrant, from SONUS COMMUNICATIONS, INC., a Virginia corporation (the
"Company"), at the price set forth in Section 8 hereof, the number of shares of
the Company's common stock, $.001 par value per share (the "Common Stock"), set
forth in Section 7 hereof. This Placement Agent Warrant, together with all
warrants issued in replacement hereof and warrants held by Holders in respect
hereof, are referred to herein as the "Warrant" and the shares of Common Stock
issued pursuant to the terms of the Warrant are sometimes referred to herein as
"Warrant Shares". The Warrant is issued in accordance with the terms of that
certain Placement Agent Agreement by and between the Company and the Placement
Agent of event date herewith.

       Section 1. Exercise of Warrant. To exercise this Warrant in whole or in
part, the Holder shall deliver to the Company at its principal office, (a) a
written notice, in substantially the form of the exercise notice attached hereto
(the "Exercise Notice"), of the Holder's election to exercise this Warrant,
which notice shall specify the number of shares of Common Stock to be purchased,
(b) a check or money wire transfer in the amount of the aggregate exercise price
for the Warrant Shares being purchased, and (c) this Warrant. The Company shall
as promptly as practicable, and in any event within twenty (20) days after
<PAGE>   2
delivery to the Company of (i) the Exercise Notice, (ii) the check mentioned
above, and (iii) this Warrant, execute and deliver or cause to be executed and
delivered, in accordance with such notice, a certificate or certificates
representing the aggregate number of shares of Common Stock specified in such
notice. If the Holder elects to purchase, at any time, less than the number of
shares of Common Stock then purchasable under the terms of this Warrant, the
Company shall issue to the Holder a new Warrant exercisable into the number of
remaining shares of Common Stock purchasable under this Warrant. Each
certificate representing Warrant Shares shall bear the legend or legends
required by applicable securities laws as well as such other legend(s) the
Company requires to be included on certificates for its Common Stock. Such
certificate or certificates shall be deemed to have been issued and such holder
or any other person so designated to be named therein shall be deemed for all
purposes to have become a holder of record of such shares as of the date the
Exercise Notice is delivered to the Company. The Company shall pay all expenses,
taxes and other charges payable in connection with the preparation, issuance and
delivery of such stock certificates except that, in case such stock certificates
shall be registered in a name or names other than the name of the Holder, funds
sufficient to pay all stock transfer taxes that are payable upon the issuance of
such stock certificate or certificates shall be paid by the Holder at the time
of delivering the Exercise Notice. All shares of Common Stock issued upon the
exercise of this Warrant shall be validly issued, fully paid, and nonassessable.
This Warrant may be exercised on multiple occasions before the expiration of its
term as described in this Section 1. This Warrant, and any warrants issued in
replacement of this Warrant, will expire on fifth anniversary of the issuance
date set forth above (the "Expiration Date")

       Section 2. Reservation of Shares. The Company hereby covenants that at
all times during the term of this Warrant there shall be reserved for issuance
such number of shares of its Common Stock as shall be required to be issued upon
exercise of this Warrant.

       Section 3. Fractional Shares. This Warrant may be exercised only for a
whole number of shares of Common Stock, and no fractional shares or scrip
representing fractional shares shall be issuable upon the exercise of this
Warrant.

       Section 4. Transfer of Warrant and Warrant Shares. The Holder may freely
sell, pledge, hypothecate, or otherwise transfer this Warrant, in whole or in
part, and any or all of the Warrant Shares; provided that any such sale, pledge,
hypothecation, or transfer is made in compliance with the Act or pursuant to an
available exemption from registration under the Act relating to the disposition
of securities, and is made in accordance with applicable State securities laws.

       Section 5. Loss of Warrant. Upon receipt by the Company of evidence
satisfactory to it of the loss, theft, or destruction of this Warrant, and of
indemnification satisfactory to it, or upon surrender and cancellation of this
Warrant, if mutilated, the Company will execute and deliver a new Warrant of
like tenor.

       Section 6. Rights of the Holder. No provision of this Warrant shall be
construed as conferring upon the Holder the right to vote, consent, receive
dividends or receive notice other than as expressly provided herein. No
provision hereof, in the absence of affirmative

                                      -2-
<PAGE>   3

action by the Holder to exercise this Warrant, and no enumeration herein of the
rights or privileges of the Holder, shall give rise to any liability of the
Holder for the purchase price of any Warrant Shares or as a stockholder of the
Company, whether such liability is asserted by the Company or by creditors of
the Company.

       Section 7. Number of Warrant Shares. This Warrant shall be exercisable
for up to 78,750 shares of the Company's Common Stock in accordance with the
terms of the Placement Agent Agreement, as adjusted as provided herein.

       Section 8. Exercise Price and Adjustment of Number of Warrant Shares
Issuable.

       (a)    Determination of Exercise Price. The per share purchase price (the
"Exercise Price") for each of the Warrant Shares purchasable under this Warrant
shall be equal to $1.00 in accordance with the Placement Agent Agreement, as
adjusted in accordance with Section 8(b) below.

       (b)    Adjustment of Exercise Price. In the event of an adjustment under
Section 8(c) or Section 9 of the number of Warrant Shares issuable upon exercise
of this Warrant, the Exercise Price shall be adjusted to an amount which is
equal to the product of: (i) the Exercise Price immediately prior to such
adjustment, and (ii) a fraction, the numerator of which is the number of Warrant
Shares issuable pursuant to this Warrant immediately after giving effect to such
adjustment, and the denominator of which is the number of Warrant Shares
issuable pursuant to this Warrant immediately prior to such adjustment.

       (c)    Adjustment to Number of Warrant Shares Issuable. If (i) the
Company at any time or from time to time after the issuance date set forth above
shall declare or pay any dividend on the Common Stock payable in Common Stock,
or effect a subdivision of the outstanding shares of Common Stock into a greater
number of shares of Common Stock (by reclassification or otherwise than by
payment of a dividend in Common Stock), or (ii) the outstanding shares of Common
Stock shall be combined or consolidated, by reclassification, reverse stock
split or otherwise, into a lesser number of shares of Common Stock, then, and
in any such event, the number of Warrant Shares which the Holder shall be
entitled to receive upon exercise of the Warrant following such event shall be
equal to the product of (i) the number of Warrant Shares which the Holder was
entitled to receive upon exercise of the Warrant prior to such event, and (ii) a
fraction, the numerator of which is the number of issued and outstanding shares
of Common Stock after giving effect to such event, and the denominator of which
is the number of issued and outstanding shares of Common Stock immediately prior
to such event.

       (d)    Adjustment of Warrant Shares for Mergers or Reorganization, etc.
In case of any consolidation or merger of the Company with or into another
corporation or the conveyance of all or substantially all of the assets of the
Company to another corporation, other than a Qualifying Transaction (as used
herein, such capitalized term shall have the meaning provided such term in the
Placement Agent Agreement), this Warrant shall be exercisable into the number of
shares of stock or other securities or property to which a holder of the number
of shares of Common Stock of the Company deliverable upon exercise

                                      -3-
<PAGE>   4

of this Warrant would have been entitled upon such consolidation, merger or
conveyance; and, in any such case, appropriate adjustment (as determined by the
Board of Directors of the Company) shall be made in the application of the
provisions herein set forth with respect to the rights and interest thereafter
of the holder of this Warrant, to the end that the provisions set forth herein
(including provisions with respect to changes in and other adjustments of the
Exercise Price) shall thereafter be applicable, as nearly as reasonable may be,
in relation to any shares of stock or other property thereafter deliverable upon
the exercise of this Warrant.

       (e)    No Impairment. The Company will not, through any reorganization,
transfer of assets, consolidation, merger, dissolution, issue or sale of
securities or any other voluntary action, avoid or seek to avoid the observance
or performance of any of the terms to be observed or performed hereunder by the
Company, but will at all times in good faith assist in the carrying out of all
the provisions of this Section 8 and in the taking of all such action as may be
necessary or appropriate in order to protect the exercise rights of the holder
of this Warrant against impairment.

       (f)    Notice of Record Date. In the event that the Company shall propose
at any time:

                     (i)    to declare any dividend or distribution upon its
Common Stock, whether in cash, property, stock or other securities, whether or
not a regular cash dividend and whether or not out of earnings or earned
surplus;

                     (ii)   to offer for subscription pro rata to the holders of
any class or series of its stock any additional shares of stock of any class or
series or other rights;

                     (iii)  to effect any reclassification or recapitalization
of its Common Stock outstanding involving a change in the Common Stock; or

                     (iv)   to merge or consolidate with or into any other
corporation, or sell, lease or convey all or substantially all of its property
or business, or to liquidate, dissolve or wind up; or

                     (v)    to conduct a public offering,

              then in connection with each such event, the Company shall send to
the holder of this Warrant at least 20 days' prior written notice of the date on
which a record shall be taken for such dividend, distribution or subscription
rights (and specifying the date on which the holders of Common Stock shall be
entitled thereto) or for determining rights to vote in respect of the matter as
referred to above. Each such written notice shall be given by first class mail,
postage prepaid, addressed to the holder of this Warrant at the address for each
such holder as shown on the books of the Company.

       (g)    Issue Taxes. The Company shall pay any and all issue and other
taxes that may be payable in respect of any issue or delivery of shares of
Common Stock on exercise of this Warrant, in whole or in part; provided,
however, that the Company shall not be

                                      -4-
<PAGE>   5

obligated to pay any transfer taxes resulting from any transfer requested by any
holder in connection with any such exercise.

       (h)    Reservation of Stock Issuable Upon Conversion. The Company shall
at all times reserve and keep available out of its authorized but unissued
shares of Common Stock, solely for the purpose of effecting the exercise of this
Warrant, such number of its shares of Common Stock as shall from time to time be
sufficient to effect the exercise of this Warrant; and if at any time the number
of authorized but unissued shares of Common Stock shall not be sufficient to
effect the exercise of this Warrant, the Company will take such corporate action
as may, in the opinion of its counsel, be necessary to increase its authorized
but unissued shares of Common Stock to such number of shares as shall be
sufficient for such purpose.

       (i)    Fractional Shares. No fractional share shall be issued upon the
exercise, in whole or in part, of this Warrant. If any exercise in whole or in
part of this Warrant would result in the issuance of a fraction of a share of
Common Stock, the Company shall, in lieu of issuing any fractional share, pay
the holder otherwise entitled to such fraction a sum in cash equal to the fair
market value of such fraction on the date of exercise (as determined in good
faith by the Board of Directors of the Company).

       (j)    Cashless Exercise. The Holder shall have the right to pay all or a
portion of the Exercise Price by making a "Cashless Exercise" pursuant to this
Section, in which case the portion of the Exercise Price to be so paid shall be
paid by reducing the number of Warrant Shares otherwise issuable pursuant to
this Warrant in accordance with the formula set forth below so that the number
of Warrant Shares to be issued to the Holder as a result of a Cashless Exercise
shall therefore be:

<TABLE>
<S>                                                                       <C>
       (Fair Market Value Per Share-Exercise Price per Warrant Share)  X  the number of Warrant Shares
       --------------------------------------------------------------         otherwise issuable
                Fair Market Value Per Share
</TABLE>

* Within ten (10) days of receipt of an election to exercise this Warrant
specifying a Cashless Exercise, the Company shall provide to the Holder in
writing its determination of the fair market value per share of Common Stock
(including the basis therefor), which shall be determined by the Board of
Directors of the Company and shall be binding on the Holder unless the Holder
objects thereto in writing within ten (10) business days of the Holder's receipt
of such determination. In the event the Company and the Holder cannot agree on
the amount of the fair market value per share of Common Stock within ten (10)
business days of the date of the Holder's delivery of its objection, such amount
shall be determined by an appraiser experienced in making such determinations
mutually selected by the Board of Directors of the Company and the Holder, the
fees and expenses of which shall be paid by the Company. (The fair market value
per share of Common Stock determined in accordance with this procedure is
referred to above as the "Fair Market Value Per Share").

       Section 9. Reclassification or Reorganization. In case of any
reclassification, capital reorganization or other change of outstanding shares
of Common Stock of the Company (other than a change in par value, or from par
value to no par value, or from no par value to par value, or as a result of an
action described more particularly in Section 8(c)

                                      -5-
<PAGE>   6

hereof), the Company shall cause effective provision to be made so that the
Holder shall have the right thereafter by exercising this Warrant, to purchase
the kind and amount of shares of stock and other securities and property
receivable upon such reclassification, capital reorganization or other change,
by a holder of the number of shares of Common Stock which might have been
purchased upon exercise of this Warrant immediately prior to such
reclassification or change. Any such provision shall include provision for
adjustments which shall be as nearly equivalent as may be practicable to the
adjustments provided for in this Warrant. The foregoing provisions of this
Section shall similarly apply to successive reclassifications, capital
reorganizations and changes of shares of Common Stock. In the event that in any
such capital reorganization, reclassification, or other change, additional
shares of Common Stock shall be issued in exchange, conversion, substitution or
payment, in whole or in part, for or of a security of the Company other than
Common Stock, any amount of the consideration received upon the issue thereof
being determined by the Board of Directors of the Company shall be final and
binding on the Holder.

       Section 10. Piggy-Back Registration Rights.

       (a)    Grant of Piggy-Back Rights. In the event that the Company shall
register any of its common stock, par value $.001 per share (a "Registered
Offering"), either for its own account or the account of any other holder or
holders of equity securities of the Company, other than (i) a registration
relating solely to employee benefit plans, (ii) a registration relating solely
to a Rule 145 transaction, (iii) a registration in which the only equity
security being registered is capital stock issuable upon conversion of
convertible (or exchange of exchangeable) debt securities which are also being
registered, or (iv) an initial public offering of the Company, the Company will
provide you with written notice thereof within 90 days of the filing date of the
first registration statement filed in connection with the Registered Offering
(the "Company Notice"), and, subject to the other terms and conditions set forth
in this Section, include in such registration (and any related qualification
under blue sky laws or other compliance) and any underwriting involved therein,
the Warrant Shares (collectively, the "Registrable Securities") specified in a
written request or requests made by the Holders to the Company within 10 days
after receipt of the Company Notice.

       (b)    Underwritten Registered Offering. If the Registered Offering of
which the Company gives notice is for a registered public offering involving an
underwriting, the Company shall so advise you as a part of the Company Notice.
In such event, your rights to registration pursuant to this Section 10 shall be
conditioned upon your participation in such underwriting, and the inclusion of
your Registrable Securities in the underwriting shall be limited to the extent
provided herein. You shall (together with the Company and the other holders
distributing their securities through such underwriting, if any) enter into an
underwriting agreement in customary form with the managing underwriter selected
for such underwriting by the Company. Notwithstanding any other provision of
this Section 10, if the managing underwriter determines that marketing factors
require a limitation of the number of shares to be underwritten, the managing
underwriter may limit the number of your Registrable Securities to be included
in such registration to such number of your Registrable Securities which the
managing underwriter determines can be included in such underwriting without
reducing the number of shares to be sold by the Company pursuant to such
underwriting. In such event, the Company shall so advise you and the number of

                                      -6-
<PAGE>   7

shares (other than shares being registered by the Company) that may be included
in the registration and underwriting shall be allocated among all the holders of
the Company's shares wishing to participate in the Registered Offering in
proportion, as nearly as practicable, to the respective amounts of shares held
by such holders at the time of filing the Registration Statement. To facilitate
the allocation of shares in accordance with the above provisions, the Company
may round the number of shares allocated to any holder to the nearest 100
shares. If you disapprove of the terms of any such underwriting, you may elect
to withdraw therefrom by written notice to the Company and the managing
underwriter. Any securities excluded or withdrawn from such underwriting shall
be withdrawn from such registration, and shall not be transferred in a public
distribution prior to 180 days after the effective date of the registration
statement relating thereto, or such other shorter period of time as the
underwriters may require.

       (c)    Termination and Withdrawal of Registration. The Company shall have
the right to terminate or withdraw any Registered Offering or other registration
prior to the effectiveness of such registration whether or not you have elected
to include your Registrable Securities in such registration.

       (d)    Expenses. All registration expenses incurred in connection with
registrations pursuant to this Section 10 shall be borne by the Company. Unless
otherwise stated, all selling expenses relating to your Registrable Securities
shall be borne by you.

       (e)    Notification Requirements. In the case of each registration,
qualification or compliance effected by the Company under this Section 10 in
which the Holder participates, the Company will keep such Holder advised in
writing as to the initiation of each registration, qualification and compliance
and as to the completion thereof. At its expense the Company will:

              (i)    to the extent required under this Agreement, prepare and
file with the Commission a registration statement with respect to such
securities and use reasonable best efforts to cause such registration statement
to become and remain effective for at least one hundred twenty (120) days or
until the distribution described in the registration statement has been
completed, whichever first occurs; and

              (ii)   furnish to such Holder, should such Holder participate in
such registration, and to the underwriters of the securities being registered
such reasonable number of copies of the registration statement, preliminary
prospectus, final prospectus and such other documents you and/or the
underwriters may reasonably request in order to facilitate the public offering
of such securities.

       (f)    Underwriting Agreement Governs. In the event the terms of this
Section 10 conflict with the terms of any underwriting agreement in connection
with any registration hereunder, the terms of such underwriting agreement shall
control.

       (g)    Information. If your Registrable Securities are to be included in
any Registered Offering, you shall furnish to the Company such information as
the Company

                                      -7-
<PAGE>   8

may request in writing and as shall be required in connection with any
registration, qualification or compliance referred to in this Agreement.

       (h)    Termination. The rights granted pursuant to this Section 10 shall
terminate at such time as the Company has registered your Registrable Securities
in a Registered Offering or other registration. Holders may exercise its
piggy-back registration rights granted under this Section 10 not more than two
(2) times.

       Section 11. Demand Registration.

       (a)    Definition of Registration Statement. "Registration Statement"
shall mean any registration statement under the Securities Act on an appropriate
form (which form shall be available for the sale of the Registrable Securities
in accordance with the intended method or methods of distribution thereof and
shall include all financial statements required by the SEC to be filed
therewith) which covers Registrable Securities pursuant to the provisions of
this Agreement, including the Prospectus included in such registration
statement, amendments (including post-effective amendments) and supplements to
such registration statement, and all exhibits to and all material incorporated
by reference in such registration statement.

       (b)    Grant of Demand Registration Rights. Upon the completion of a
Qualifying Transaction, the Holders of Registrable Securities constituting at
least fifty one percent (51%) of the aggregate number of (i) Registrable
Securities issued upon exercise of the Warrants granted pursuant to the
Consulting Agreement entered into by and between the Company and L. Flomenhaft &
Co., Incorporated, and (ii) securities issued upon exercise of the warrants
granted and to be granted pursuant to that certain Placement Agent Agreement
entered into by and between the Company and L. Flomenhaft & Co., Incorporated,
may request at any time that the Company file a registration statement under the
Securities Act on an appropriate form (which form shall be available for the
sale of the Registrable Securities in accordance with the intended method or
methods of distribution thereof and shall include all financial statements
required by the SEC to be filed therewith), other than Form S-1 (a "Registration
Statement"), covering the shares of Registrable Securities that are the subject
of such request; provided, however, that the Company shall not become obligated
to file any Registration Statement until the earlier of (i) three months
following the date of completion of the Qualifying Transaction, or (ii) the
first date on which the Company may file such a registration statement in
accordance with all applicable State and Federal securities laws.

       (c)    Number of Demand Registrations. The Company shall be obligated to
prepare, file and cause to become effective pursuant to this Section no more
than one Registration Statement; provided, however, that a Registration
Statement shall not be counted hereunder unless it becomes effective and is
maintained effective in accordance with the requirements specified in this
Section.

       (d)    Required Thresholds. The Company shall not be obligated to
prepare, file and cause to become effective pursuant to this Section a
Registration Statement on Form S-1. The Company shall not be obligated to
prepare, file and cause to become effective any

                                      -8-
<PAGE>   9

Registration Statement if such demand is made less than 90 days after the
effective date of the Company's most recent registration statement for shares of
Common Stock (other than a Registration Statement on Form S-4 or Form S-8 or any
successor forms thereto).

       (e)    Underwriter's Cutback. If the public offering of Registrable
Securities is to be underwritten and, in the good faith judgment of the managing
underwriter, the inclusion of all the Registrable Securities requested to be
registered hereunder would interfere with the successful marketing of a smaller
number of such shares of Registrable Securities, the number of shares of
Registrable Securities to be included shall be reduced (except for shares of
Registrable Securities offered by the Company) to such smaller number with the
participation in such offering to be pro rata among the Holders of Registrable
Securities other than the Company requesting such registration, based upon the
number of shares of Registrable Securities owned by such Holders. Any shares
that are thereby excluded from the offering shall be withheld from the market by
the Holders thereof for a period (not to exceed 30 days prior to the effective
date and 75 days thereafter) that the managing underwriter reasonably determines
is necessary in order to effect the underwritten public offering.

       (f)    Managing Underwriter. The managing underwriter or underwriters of
any underwritten public offering covered by a Demand Registration shall be
selected by the Company.

       (g)    Black-Out Periods of Investor. Notwithstanding anything herein to
the contrary, (i) the Company shall have the right, exercisable once, to require
the Holders not to sell under a Demand Registration or to suspend the
effectiveness thereof (but not for a period exceeding 90 days in any calendar
year) if the Company determines, in its good faith judgment, that such offering
or continued effectiveness would interfere with any material financing,
acquisition, disposition, corporate reorganization or other material transaction
involving the Company or any of its subsidiaries or public disclosure thereof
would be required prior to the time such disclosure might otherwise be required,
or when the Company is in possession of material information that it deems
advisable not to disclose in a registration statement.

       (h)    Expenses. All registration expenses incurred in connection with
registrations pursuant to this Section 11 shall be borne by the Company. Unless
otherwise stated, all selling expenses relating to your Registrable Securities
shall be borne by you.

       (i)    Underwriting Agreement Governs. In the event the terms of this
Section 11 conflict with the terms of any underwriting agreement in connection
with any registration hereunder, the terms of such underwriting agreement shall
control.

       (j)    Information. If your Registrable Securities are to be included in
any Registered Offering, you shall furnish to the Company such information as
the Company may request in writing and as shall be required in connection with
any registration, qualification or compliance referred to in this Agreement.

                                      -9-
<PAGE>   10

       (k)    Termination. The rights granted pursuant to this Section 11 shall
terminate at such time as the Company has registered your Registrable Securities
in a Registered Offering or other registration. The demand registration rights
granted under this Section 11 may be exercised only by demand of Holders of at
least fifty one percent (51%) of the sum of (i) Warrant Shares which are
Registrable Securities, and (ii) shares of Common Stock of the Company issuable
upon exercise of the warrants granted pursuant to the Consulting Agreement. The
demand registration rights granted under this Section 11 and the registration
rights granted pursuant to the warrants granted in connection with the
Consulting Agreement, including, without limitiation, Section 11 of the
Consulting Warrant of even date herewith, shall all be deemed to be exercised
simultaneously by a proper exercise under either the this Warrant or the
Consulting Warrant, and may be exercised not more than once in the aggregate.

       Section 12. Indemnification.

       (a)    Indemnification. The Holders agree, if any of Holders' Registrable
Securities are included in the securities as to which such registration,
qualification or compliance is being effected, to indemnify the Company, each of
its directors and officers, each underwriter, if any, of the Company's
securities covered by such a registration statement, each Person who controls
the Company or such underwriter within the meaning of Section 15 of the
Securities Act, and each other such holder, each of its officers and directors
and each Person controlling such holder within the meaning of Section 15 of the
Securities Act, against all claims, losses, damages and liabilities (or actions
in respect thereof) arising out of or based on any untrue statement (or alleged
untrue statement) of a material fact contained in any such registration
statement, prospectus, offering circular or other document, or any omission (or
alleged omission) to state therein a material fact required to be stated therein
or necessary to make the statements therein not misleading, and will reimburse
the Company, such holders, such directors, officers, Persons, underwriters or
control Persons for any legal or any other expenses reasonably incurred, as such
expenses are incurred, in connection with investigating or defending any such
claim, loss, damage, liability or action, in each case to the extent, but only
to the extent, that such untrue statement (or alleged untrue statement) or
omission (or alleged omission) is made in such registration statement,
prospectus, offering circular or other document in reliance upon and in
conformity with written information furnished to the Company by you.
Notwithstanding the foregoing, your liability under this subsection shall be
limited in an amount equal to the initial price of the Registrable Securities
sold by you, unless such liability arises out of or is based on willful
misconduct by you.

              (g)    Indemnification Procedure. Each party entitled to
indemnification under this Section (the "Indemnified Party") shall give notice
to the party required to provide indemnification (the "Indemnifying Party")
after such Indemnified Party has actual knowledge of any claim as to which
indemnity may be sought, and shall permit the Indemnifying Party to assume the
defense of any such claim or any litigation resulting therefrom, provided that
counsel for the Indemnifying Party, who shall conduct the defense of such claim
or litigation, shall be approved by the Indemnified Party (whose approval shall
not be unreasonably withheld), and the Indemnified Party may participate in such
defense at such party's expense, and provided further that the failure of any
Indemnified

                                      -10-
<PAGE>   11

Party to give notice as provided herein shall not relieve the Indemnifying Party
of its obligations under this Agreement, unless the failure to give such notice
is materially prejudicial to an Indemnifying Party's ability to defend such
action, and provided further that the Indemnifying Party shall not assume the
defense for matters as to which there is a conflict of interest or separate and
different defenses. No Indemnifying Party, in the defense of any such claim or
litigation, shall, except with the consent of each Indemnified Party, consent to
entry of any judgment or enter into any settlement which does not include as an
unconditional term thereof the giving by the claimant or plaintiff to such
Indemnified Party of a release from all liability in respect to such claim or
litigation.

       Section 13. Redemption. The Company shall have the authority to redeem
this Warrant at any time.

                                      -11-
<PAGE>   12

       Section 14. Applicable Law. This Warrant shall be construed in accordance
with the laws of the Commonwealth of Virginia without giving effect to the
conflicts of law provisions of such laws.

                                              SONUS COMMUNICATIONS, INC.

                                              By:
                                                   --------------------
                                                   Charles W. Albo
                                                   Chief Executive Officer

                                      -12-
<PAGE>   13

EXERCISE NOTICE

Dated:
       --------------------

       The undersigned hereby irrevocably elects to exercise his, her or its
right to purchase _________ shares of the common stock, $__ par value per share
(the "Common Stock"), of SONUS COMMUNICATIONS, INC., a Virginia corporation (the
"Company"), such right being pursuant to a Warrant dated _________, 1998, and as
issued to the undersigned by the Company, and remits herewith the sum of $______
in payment for same in accordance with the Exercise Price specified in Section 8
of said Warrant.

INSTRUCTIONS FOR REGISTRATION OF STOCK

Name:
                                                     -------------------------
     (Please typewrite or print in block letters)

Address:
         ---------------------------------

Signature:
           ------------------------------

<PAGE>   14

ASSIGNMENT FORM

Dated:
       ---------------------

       For value received ____________________ hereby sells, assigns and
transfers unto

       Name:
                                        ---------------------------------
             (Please typewrite or print block letters)

       Address:
                 ---------------------------------
                 ---------------------------------

  and appoints:
                 ---------------------------------
                 ---------------------------------

Attorney to transfer the said Warrant on the books of SONUS COMMUNICATIONS, INC.
with full power of substitution in the premises.

                                       Signature:
                                                  --------------------------

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