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                                                                   Exhibit 10.13


                              AGREEMENT OF SUBLEASE

         THIS AGREEMENT OF SUBLEASE is made as of the 14th day of April,
2000, by and between Unisphere Solutions, Inc., formerly known as Argon
Networks, Inc., a Delaware corporation ("Landlord"), having an office at 200
Wheeler Road, Burlington, MA 01803 and Sonus Networks, Inc. ("Tenant"), a
Delaware corporation, having an office at 5 Carlisle Road, Westford, MA 01886.

                                     WITNESS

         WHEREAS, by Lease dated as of May 16, 1997, as amended by that certain
First Amendment to Lease dated as of June 22, 1998 (the "First Amendment")
(collectively hereinafter the "Prime Lease") by and between MGI 25 Porter Road,
Inc. ("MGI") and Landlord, MGI leased to Landlord the premises (as hereinafter
defined) (the "Premises") which is located within the building known by the
street address of 25 Porter Road, Littleton, MA 01460 (the "Building"), which
Premises are more particularly described in the Prime Lease and include 22,581
rentable square feet on the second floor (2nd) floor of the Building as
indicated by crosshatching on the floor plan attached to the Prime Lease as
Exhibit A (a copy of such Prime Lease is attached hereto as Exhibit A and made a
part hereof);

         WHEREAS, BCIA New England Holdings, LLC ("Prime Landlord") is the
successor in interest to MGI; and

         WHEREAS, Landlord desires to sublease to Tenant and Tenant desires to
sublease from Landlord the Premises, and Landlord is willing to sublease the
Premises on the terms and conditions hereinafter set forth.

         NOW, THEREFORE, in consideration of the mutual covenants herein
contained and for other good and valuable consideration, the parties agree as
follows:

         1. SUBLEASING OF PREMISES. Subject to the written consent of the Prime
Landlord, Landlord hereby subleases to Tenant and Tenant hereby subleases from
Landlord the Premises, upon and subject to all of the terms, covenants, recitals
and conditions hereinafter set forth.

         2. TERM The term (the "Term") of this Sublease shall commence on May 1,
2000 (the "Commencement Date"), with said Term to expire at midnight on May 30,
2003, (the "Expiration Date"), unless sooner terminated as hereinafter provided.

         3. BASE RENT. During the Term, Tenant shall pay to Landlord, in lawful
money of the United States which at the time shall be legal tender in payment of
all debts and dues, public and private, an annual fixed rent (the "Base Rent")
as follows: for the period commencing on the Commencement Date through
May 31, 2002, Tenant shall pay a Base


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Rent of $237,100.50, payable in equal monthly installments of $19,758.38; and
for the period commencing on June 1, 2002 through May 30, 2003, Tenant shall pay
a Base Rent of $258,552.45, payable in equal monthly installments of $21,546.04.
All such monthly installments shall be paid in advance, on the first (lst) day
of each month during the Term, at the office of the Landlord, or such other
place as Landlord may designate, without any setoff or deduction of any kind
whatsoever.

         4.       ADDITIONAL RENT.

                  (a) Beginning on the Commencement Date, Tenant shall pay , as
Additional Rent, without notice or demand, Tenant's Proportionate Share of Taxes
and Operating Expenses (as such terms are defined in the Prime Lease) in
accordance with Section 2.4.6 of the Prime Lease. Rental and any other sums due
hereunder not paid by the due date shall bear interest and be subject to late
payment fees, all in accordance with the provisions of Section 17.7 of the Prime
Lease. All payments shall be made to Landlord at its address set forth in
Section 24 below, or at such other address or addresses as Landlord may from
time to time designate by written notice to Tenant.

                  Tenant shall also pay, as Additional Rent, the cost of all
utilities furnished to Tenant on the Premises, including, but not limited to,
electricity, gas, oil, water and sewer. Tenant agrees to pay any and all such
charges for the Premises to Landlord in the event any such utilities are not
separately metered to Tenant or directly to the utility company if such
utilities are separately metered. Electricity usage within the Premises shall be
measured by the use of a check-meter, and Tenant shall pay to Landlord monthly
the amount invoiced by Landlord for electricity used in the Premises as
indicated by such check-meter, at the rate then being charged by the local
electrical utility company.

                  (b) Tenant's obligation to pay Additional Rent hereunder shall
be on account of the period from and after the Rent Commencement Date and shall
survive the Expiration Date or sooner termination of the Term.

                  (c) All amounts payable by Tenant to Landlord pursuant to this
Sublease, including, without limitation, Base Rent and Additional Rent, shall be
deemed and constitute rent and, in the event of any non-payment thereof,
Landlord shall have all of the rights and remedies provided herein, in the Prime
Lease or in law or at equity for non-payment of rent.

                  (d) Tenant agrees to pay to Landlord Tenant's pro rata share
of any real estate taxes attributable to leasehold improvements within the
Premises which are payable by Landlord under the Prime Lease.

         5.       CARE, SURRENDER AND RESTORATION OF THE PREMISES.

                  (a) Without limiting any other provision of this Sublease or
the Prime Lease, Tenant shall take good care of the Premises, suffer no waste or
injury thereto and shall comply with all those laws, orders and regulations
applicable to the Premises, the Building and Tenant's use or manner of use
thereof, which are imposed on Landlord, as


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tenant under the Prime Lease, in connection with the Premises and the Building,
including without limitation the Rules and Regulations which are attached to the
Prime Lease as Exhibit G.

                  (b) At the expiration or other termination of the Term, Tenant
shall surrender the Premises and all alterations and additions thereto
(including any fixtures, panelling, railings and like installation installed at
the Premises at any time by Tenant, by Prime Landlord or by Landlord) in good
order, repair and condition, ordinary wear and tear and damage by casualty only
excepted, first removing all goods and effects of Tenant and, to the extent
specified by Landlord by notice to Tenant given at least thirty (30) days before
such expiration or termination, all alterations and additions made by or on
behalf of Tenant. Tenant shall repair any damage caused by such removal and
restore the Premises and leave them clean and neat in compliance with the
requirements of Section 5 of this Sublease and Section 6.1 of the Prime Lease.
All property permitted or required to be removed by Tenant upon the Expiration
Date or sooner termination of the Term remaining in the Premises shall be deemed
abandoned and may, at the election of Landlord, either be retained as Landlord's
property or may be removed from the Premises by Landlord, at Tenant's expense.
Any such reasonable expenses shall be paid by Tenant to Landlord upon demand
therefor and shall be deemed Additional Rent, collectible by Landlord in the
same manner and with the same remedies as though said sums were Base Rent
reserved hereunder.

         (c) Upon the Expiration Date or sooner termination of the Term, Tenant
shall quit and surrender the Premises to Landlord, broom clean, in good order
and condition, ordinary wear and tear and damage by fire and other casualty
excepted, and Tenant shall remove all of its property. If the Expiration Date or
sooner termination of the Term of this Sublease falls on a Sunday, this Sublease
shall expire at noon on the preceding Saturday unless it be a legal holiday, in
which case it shall expire at noon on the preceding business day. Tenant shall
observe and perform the covenants herein stated and Tenant's obligations
hereunder shall survive the Expiration Date or sooner termination of the Term.

         6.       USE. Tenant shall use and occupy the Premises for the purposes
permitted under Section 4.1 of the Prime Lease including light assembly of
networking gear, that is, installation of components into a case/chassis,
including testing, and for no other purpose.

         7.       SUBORDINATION TO AND INCORPORATION OF TERMS OF PRIME LEASE.

                  (a) This Sublease is in all respects subject and subordinate
to the terms and conditions of the Prime Lease and to the matters to which the
Prime Lease is or shall be subordinate. Except as otherwise expressly provided
in this Sublease, the terms, provisions, covenants, stipulations, conditions,
rights, obligations, remedies and agreements of the Prime Lease are incorporated
in this Sublease by reference and made a part hereof as if herein set forth at
length, and shall, as between Landlord and Tenant (as if they were the Landlord
and Tenant, respectively, under the Prime Lease and as if the Premises being
sublet hereby were the Prime Lease Premises demised under the Prime Lease),
constitute the terms of this Sublease, except to the extent that they do not
relate to the Premises or are inapplicable to, inconsistent with, or modified or
eliminated by, the terms of this Sublease.


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In particular, it is intended that Tenant shall not be subject to duplicate
monetary obligations to Landlord and Prime Landlord. To the extent that Tenant
is required by this Sublease to make monetary payments to Landlord (such as for
rent, additional rent or upon default) Tenant shall not be obligated to the
Prime Landlord for any such monetary obligations nor to Landlord for its
monetary obligations to the Prime Landlord. Landlord and Tenant acknowledge and
agree that Tenant has reviewed and is familiar with the Prime Lease and Landlord
hereby represents that the copy delivered to Tenant for such purpose and
attached hereto as Exhibit A is a true, correct and complete copy of such Prime
Lease.

                  (b) In the event of a default by Landlord, as tenant under the
Prime Lease, resulting in the termination, reentry or dispossession thereunder,
Prime Landlord shall take over all of the right, title and interest of Landlord
under this Sublease and Tenant hereunder shall, at the option of the Prime
Landlord, attorn to and recognize Prime Landlord as Landlord hereunder except
that Prime Landlord shall not (i) be liable for any previous act or omission of
Landlord under this Sublease, (ii) be subject to any offset, not expressly
provided for in this Sublease, which theretofore accrued to Tenant against
Landlord, or (iii) be bound by any previous modification of this Sublease or by
any previous prepayment of more than one month's rent, and shall, promptly upon
Prime Landlord's request, execute and deliver all instruments necessary or
appropriate to confirm such attornment and recognition. Tenant hereby waives all
rights under any present or future law to elect, by reason of the termination of
such Prime Lease, to terminate this Sublease or surrender possession of the
Premises.

         8.       TENANT'S OBLIGATIONS. Except as otherwise specifically
provided herein, during the term of this Sublease all acts to be performed and
all of the terms, provisions, covenants, stipulations, conditions, rights,
obligations, remedies and agreements to be observed by and inuring to the
benefit of, Landlord, as tenant under the Prime Lease of the Premises and
arising from and after the Commencement Date, shall be performed, and observed
by, and shall inure to the benefit of, Tenant, and Tenant's obligations shall
run to Landlord or the Prime Landlord as Landlord may determine to be
appropriate or required by the respective interests of Landlord and Prime
Landlord. Tenant shall indemnify Landlord against, and hold Landlord harmless
from and against, all costs, damages, claims, liabilities, liens and expenses
(including, but not limited to, reasonable attorneys' fees and disbursements,
court costs and other expenses of litigation or arbitration) paid, suffered,
incurred by or claimed against Landlord as a result of the nonperformance or
non-observance by Tenant, Tenant's agents, contractors, employees, invitees or
licensees of any such terms, provisions, covenants, stipulations, conditions,
obligations and agreements contained in the Prime Lease. In furtherance of the
foregoing, Tenant shall not (i) do or permit to be done anything prohibited to
Landlord, as tenant under the Prime Lease, or (ii) take any action or do or
permit anything which would result in any additional cost or other liability to
Landlord under the Prime Lease and/or this Sublease. In the event of any
inconsistency between the Prime Lease and this Sublease, such inconsistency (i)
if it relates to obligations of, or restrictions on, Tenant, shall be resolved
in favor of that obligation which is more onerous to Tenant or that restriction
which is more restrictive of Tenant, as the case may be, or (ii) if it relates
to the rights of, or benefits to be conferred upon, Tenant, shall be resolved in
favor of this Sublease.

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         9.       LANDLORD'S OBLIGATIONS. Anything contained in this Sublease or
in the Prime Lease to the contrary notwithstanding, Landlord shall have no
responsibility to Tenant for, and shall not be required to provide, any of the
services or make any of the repairs or restorations that Prime Landlord has
agreed to make or provide, or cause to be made or provided, under the Prime
Lease (including, without limitation, those set forth in Sections 3.1
[Landlord's Work], 19.3 [Landlord's Services], 4 of the First Amendment
[Preparation of Premises] thereof), and Tenant shall rely upon, and look solely
to, Prime Landlord for the provision or making thereof. If Prime Landlord shall
default in the performance of any of its obligations under the Prime Lease, or
if Tenant wishes to file a protest or to dispute any matter or thing, Landlord
has the right to protest or dispute as tenant under the Prime Lease, then Tenant
shall advise Landlord of such protest or dispute (together with all material
facts and circumstances pertaining thereto) and Landlord shall make demand on
Prime Landlord and shall employ all reasonable efforts to cause Prime Landlord
to cure such default or resolve such dispute. Except as may result from a
default of Landlord from its obligations specified in the preceding sentence,
Tenant shall not make any claim against Landlord for any damage which may arise,
nor shall Tenant's obligations hereunder be impaired or abated by reason of (i)
the failure of Prime Landlord to keep, observe or perform its obligations
pursuant to the Prime Lease, or (ii) the acts or omissions of Prime Landlord and
each of its agents, contractors, servants, employees, invitees or licensees.

         10.      COVENANTS WITH RESPECT TO THE PRIME LEASE. Tenant covenants
and agrees that Tenant shall not do anything that would constitute a default
under the Prime Lease or omit to do anything that Tenant is obligated to do
under the terms of this Sublease so as to cause there to be a default under the
Prime Lease.

         11.      BROKER. Tenant represents and warrants to Landlord that Tenant
has not dealt with any broker in connection with this Sublease. Tenant shall
indemnify Landlord against, and hold Landlord harmless from, any claim on, or
liability to, any broker or any other party with whom Tenant shall have dealt in
connection with this transaction and Sublease.

         12.      INDEMNIFICATION.

                  12.1     RECIPROCAL INDEMNIFICATION OF LANDLORD AND TENANT.

                           (a) Tenant shall indemnify, defend with competent and
experienced counsel and hold harmless Landlord from and against any and all
damages, liabilities, actions, causes of action, suits, claims, demands, losses,
costs and expenses (including without limitation reasonable attorneys' fees and
disbursements and court costs) to the extent arising from or in connection with
the negligence or willful misconduct of Tenant, its agents, employees,
representatives or contractors.

                           (b) Landlord shall indemnify, defend with competent
and experienced counsel and hold harmless Tenant, its subsidiaries and
affiliates and their respective officers, directors, shareholders and employees,
from and against any and all


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                                     - 6 -


damages, liabilities, actions, causes of action, suits, claims, demands, losses,
costs and expenses (including without limitation reasonable attorneys' fees and
disbursements and court costs) to the extent arising from or in connection with
the negligence or willful misconduct of Landlord, its agents, employees,
representatives or contractors.

                           (c) The party seeking indemnification under this
Section (the "Indemnified party") shall provide prompt written notice of any
third party claim to the party from whom indemnification is sought (the
"Indemnifying Party"). The Indemnifying Party shall have the right to assume
exclusive control of the defense of such claim or at the option of the
Indemnifying Party, to settle the same. The Indemnified Party agrees to
cooperate reasonably with the Indemnifying Party in connection with the
performance of the Indemnifying Party's obligations under this Section.

                           (d) Notwithstanding anything to the contrary
contained in this Sublease, neither party hereto shall be liable to the other
for any indirect, special, consequential or incidental damages (including
without limitation loss of profits, loss of use or loss of goodwill) regardless
of (i) the negligence (either sole or concurrent) of either party or (ii)
whether either party has been informed of the possibility of such damages. It is
expressly understood and agreed that damages payable by either party to Prime
Landlord shall be deemed to constitute direct damages of such party.

                  12.2 INDEMNIFICATION BY TENANT OF PRIME LANDLORD. Tenant
agrees to defend, save harmless and indemnify Prime Landlord to the same extent
as Landlord is required to do so under the provisions of Section 13.2 of the
Prime Lease.

                  12.3 SURVIVAL. The provisions of this Section shall survive
the expiration or earlier termination of this Sublease.

         13.      QUIET ENJOYMENT. Subject to the terms and conditions hereof
and as long as Tenant pays all of the Base Rent and Additional Rent due
hereunder and otherwise performs and observes all of the obligations, terms and
conditions contained herein and in the Prime Lease as herein incorporated,
Tenant shall peaceably and quietly have, hold and enjoy the Premises.

         14.      TERMINATION OF PRIME LEASE. If for any reason the term of the
Prime Lease is terminated prior to the Expiration Date of this Sublease, this
Sublease shall thereupon terminate, and Landlord shall not be liable to Tenant
by reason thereof unless such termination is due solely to an event of default
on behalf of Landlord. Notwithstanding the foregoing, if the termination of the
Prime Lease does not result in the termination of this Sublease by reason of
Tenant's attornment to, and recognition of, Prime Landlord as landlord hereunder
in accordance with the provisions of Section 7(b) hereof, Landlord shall not be
liable to Tenant hereunder for damages or otherwise, and Landlord's obligation
to Tenant shall be limited to returning to Tenant a portion of any rent paid in
advance by Tenant, if any, prorated as of the date of such termination.

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         15.      MODIFICATION OF PRIME LEASE. For the purposes hereof, the
terms of the Prime Lease are subject to the following modifications:

                  (a) In all provisions of the Prime Lease requiring the
approval or consent of Prime Landlord, Tenant shall be required to obtain the
approval or consent of both Prime Landlord and Landlord. In all provisions of
the Prime Lease requiring that notice be given to Prime Landlord, Tenant shall
be required to give notice to both the Prime Landlord and Landlord.

                  (b) The following provisions of the Prime Lease do not apply
to Tenant and are hereby deleted in respect of this Sublease: Sections 2.2.1,
2.5, 2.5.1, 3.1(b), 3.2, 5.1.1, 5.2, and Article XXII.

         16.      CONSENTS. Landlord's refusal to consent to or approve any
matter or thing, whenever Landlord's consent or approval is required under this
Sublease or under the Prime Lease, as incorporated herein, shall be deemed
reasonable if Prime Landlord has refused or failed to give its consent or
approval to such matter or thing.

         17.      CONDITION OF THE PREMISES; TENANT'S CHANGES.

                  (a) Tenant represents it has made a thorough examination of
the Premises and it is familiar with the condition thereof. Tenant acknowledges
that it enters into this Sublease without any representation or warranties by
Landlord except as set forth in this Lease, or anyone acting or purporting to
act on behalf of Landlord, as to the present or future conditions of the
Premises or the appurtenances thereto or any improvements therein or of the
Building. It is further agreed that Tenant does and will accept the Premises "as
is" in their present condition and Landlord has no obligation to perform any
work therein.

                  (b) Notwithstanding anything to the contrary contained in the
Prime Lease, Tenant shall not make any changes to the Premises whatsoever,
including, without limitation, structural or non-structural changes, without the
prior written consent of Landlord which consent shall not be unreasonably
withheld, conditioned or delayed but subject to prior consent of Prime Landlord
in accordance with the Prime Lease.

         18.      ASSIGNMENT AND SUBLETTING.

                  (a) Tenant, for itself, its successors and assigns, expressly
covenants that it shall not assign, whether by operation of law or otherwise,
mortgage or pledge or otherwise transfer or encumber this Sublease, or sublet
all or any part of the Premises without obtaining the prior written consent of
Landlord, which consent shall not be unreasonably withheld with respect to
assignments or transfers to Affiliated Entities (as that term is used in the
Prime Lease), but in any other event such consent may be given or withheld in
Landlord's sole discretion. In the event that Tenant desires to vacate the
Premises during the term of this Sublease, if Tenant delivers to Landlord a
qualified alternate tenant, Landlord shall agree to consider in good faith an
arrangement to lease the Premises to the alternate tenant subject to Tenant
reimbursing Landlord for the reasonable


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cost of arranging the new lease, it being specifically understood and agreed
that Landlord has no obligation to consent to such an alternate tenant.
Notwithstanding the immediately preceding sentence, any steps taken by Landlord
or Tenant in this regard shall be subject to all terms and conditions of the
Prime Lease. The parties acknowledge that Tenant may become a public company
during the Term hereof and further acknowledge that the same will not constitute
an assignment, transfer or encumbrance of this Sublease. Landlord reserves the
right to transfer and assign its interest in and to this Sublease to any entity
or person who shall succeed to Landlord's interest in and to the Prime Lease.

                  (b) Consent by Landlord to any assignment, transfer or
subletting to any party shall not be construed as a waiver or release of Tenant
from the terms of any covenant or its primary responsibility under this
Sublease, nor shall consent to one assignment, transfer or sublease to any
person, partnership, firm or corporation be deemed to be a consent to any
subsequent assignment, transfer or subletting to another person, partnership,
firm or corporation.

         19.      INSURANCE.

                  (a) Tenant agrees to maintain all insurance coverages
specified in Section 11.2 of the Prime Lease (including without limitation
commercial general liability and property damage insurance, casualty insurance
and workers' compensation insurance) in accordance with Sections 11.2 and 1.1 of
the Prime Lease). All such insurance shall be underwritten by a company or
companies licensed to do insurance business in the Commonwealth of Massachusetts
by the Department of Insurance and in good standing, and shall be written on an
"occurrence basis." All such insurance policies shall name Landlord and Prime
Landlord as additional insureds thereunder and, in addition, shall name as
additional insureds the holders of any Mortgage of the Property of which Tenant
is notified in writing, as their respective interests may appear. Tenant shall
furnish Landlord receipts evidencing payment of the premiums for such insurance
(if requested by Landlord) and shall deposit with Landlord certificates for such
insurance no later than the Commencement Date and at least fifteen (15) days
before each insurance renewal date thereof, bearing the endorsement that the
polices will not be canceled nor will coverages be reduced until after ten (10)
days' prior written notice to both Landlord and Prime Landlord of such proposed
action. Tenant shall pay all premiums and charges for such insurance, and if
Tenant shall fail to obtain such insurance, Landlord may, but shall not be
obligated to, obtain the same, in which event the amount of the premium paid
shall be paid by Tenant to Landlord upon Landlord's demand therefor, shall be
deemed Additional Rent and shall be collectible by Landlord in the same manner
and with the same remedies as though said sums were Additional Rent reserved
hereunder.

                  (b) Tenant acknowledges that Landlord will not carry any
insurance in favor of Tenant, and that neither Prime Landlord nor Landlord will
carry insurance on Tenant's furniture and/or furnishings or any fixtures or
equipment, improvements or appurtenances of Tenant in or about the Premises.

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                                     - 9 -


         20.      WAIVER OF SUBROGATION.

                  (a) Any casualty insurance carried by the Tenant with respect
to the Premises, the Building or the Property, or property therein or
occurrences thereon shall include a clause or endorsement denying to the insurer
rights of subrogation against the other party to the extent rights have been
waived by the insured prior to occurrence of injury or loss, provided that such
clause or endorsement is obtainable without payment of an additional premium. If
such clause or endorsement is obtainable upon payment of an additional premium,
notice thereof shall be given to the Landlord and the Landlord may request the
Tenant to obtain it and shall reimburse the Tenant for the cost of such
additional premium.

                  (b) Each party, notwithstanding any provisions of this
Sublease to the contrary, hereby waives any rights of recovery against the other
for injury or loss due to hazards covered by such insurance to the extent such
party's policy permits such waivers of subrogation and then only with respect to
sums which are collectible thereunder. Landlord shall be afforded the protection
of this provision notwithstanding any right which Tenant may have to self
insure.

         21.      END OF TERM. If Tenant shall remain in possession of the
Premises or any part thereof after the expiration or prior termination of the
Term hereof, as the same may be extended, the parties agree that no such holding
over by Tenant shall operate to extend or renew this Sublease, and that any such
holding over shall cause Tenant to become a month-to-month tenant and Tenant
shall be obligated to pay monthly installment of Base Rent and Additional Rent
in an amount equal to one hundred fifty percent (150%) times the sum of the
installment of Base Rent and Estimated Additional Rent Payments payable during
the last full calendar month of the Lease Term, and such tenancy shall otherwise
be subject to all the terms, conditions, covenants and agreements of this
Sublease. Tenant further agrees to pay to Landlord any additional amounts
payable by Landlord to Prime Landlord under the Prime Lease by reason of any
such holding over by Tenant.

         22.      DEFAULT.

                  (a) In the event that Tenant shall default in the payment of
Base Rent, Additional Rent or any other charge payable hereunder within five (5)
days after written notice, or shall default in the performance or observance of
any of the terms, conditions and covenants of this Sublease within twenty (20)
days after written notice, or, if not curable within said twenty day period,
Tenant does not commence to cure said default within said twenty day period and
diligently prosecute the same to completion, and provided that there exists no
event of default under the Prime Lease, Landlord, in addition to and not in
limitation of any rights otherwise available to it, shall have the same rights
and remedies with respect to such default as are provided to Prime Landlord
under the Prime Lease with respect to defaults by Landlord as tenant thereunder,
with the same force and effect as though all such provisions relating to any
such default or defaults were set forth herein in their entirety, and Tenant
shall have all of the obligations of the tenant under the Prime Lease with
respect to such default or defaults.

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                                     - 10 -


                  (b) In the event of a default by Tenant in the performance of
any of its non-monetary obligations hereunder, including those under the Prime
Lease, Landlord may, at its option, and without waiving any other remedies for
such default herein or at law or by incorporation by reference of the Prime
Lease provided, at any time thereafter, give written notice to Tenant that if
such default is not cured, or the cure not commenced, within ten (10) days after
notice, and if so commenced is not thereafter pursued diligently to completion,
Landlord may cure such default for the account of Tenant, and any amount paid or
incurred by Landlord in so doing shall be deemed paid or incurred for the
account of Tenant and Tenant agrees promptly to reimburse Landlord therefor and
save Landlord harmless therefrom; provided, however, that Landlord may cure any
such default as aforesaid prior to the expiration of any waiting period if
reasonably necessary to protect Landlord's interest under the Prime Lease or to
prevent injury or damage to persons or property.

         23.      DESTRUCTION, FIRE AND OTHER CASUALTY. If the whole or any part
of the Premises or the Building shall be damaged by fire or other casualty and
the Prime Lease is not terminated on account thereof by either Landlord or Prime
Landlord in accordance with the terms thereof or if this Sublease is not
terminated by Tenant as if the termination provisions of the Prime Lease were
set forth herein as between Tenant and Landlord, this Sublease shall remain in
full force and effect and Base Rent and Additional Rent shall not abate except
to the extent Base Rent and Additional Rent for the Premises shall abate under
the terms of the Prime Lease.

         24.      NOTICES.

                  (a) Whenever, by the terms of this Sublease, notice, demand or
other communication shall or may be given to either party, the same shall be in
writing and addressed as follows:

                      If to Landlord:  Unisphere Solutions, Inc.
                                       200 Wheeler Road
                                       Burlington, MA  01803
                                          Attn:  Suzanne M. Zabitchuck
                                                 General Counsel

                      with a copy to:

                                       Gadsby & Hannah LLP
                                       225 Franklin Street
                                       Boston, MA  02110
                                          Attn:  Cynthia B. Keliher, Esq.

                      If to Tenant:    Sonus Networks, Inc.
                                       5 Carlisle Road
                                       Westford, MA  01886
                                       Attn:  Stephen J. Nill, Vice President of
                                              Finance and CFO


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                                     - 11 -


or to such other address or addresses as shall from time to time be designated
by written notice by either party to the other as herein provided. All notices
shall be sent by registered or certified mail, postage prepaid and return
receipt requested, or by Federal Express or other comparable courier providing
proof of delivery, and shall be deemed duly given and received (i) if mailed, on
the third business day following the mailing thereof, or (ii) if sent by
courier, the date of its receipt (or, if such day is not a business day, the
next succeeding business day). Landlord and Tenant each promptly shall deliver
to the other copies of all notices, requests, demands or other communications
which relate to the Premises or the use or occupancy thereof after receipt of
the same from Prime Landlord or others.

                  (b) Each party hereunder shall promptly furnish the other with
copies of all notices under the Prime Lease or this Sublease with respect to the
Premises which such party shall receive from Prime Landlord under the Prime
Lease.

         25.      SUBLEASE CONDITIONAL UPON CERTAIN CONSENTS. Landlord and
Tenant each acknowledge and agree that this Sublease is subject to Landlord's
obtaining the unconditional consent of Prime Landlord in accordance with the
terms of the Prime Lease, and that if such consent shall not be obtained within
fifteen (15) days of the date hereof, then this Sublease shall be deemed
cancelled and terminated and neither of the parties hereto shall have any
liability to the other.

         26.      SECURITY DEPOSIT. Tenant concurrently with the execution of
this Sublease has deposited with Landlord a deposit (the "Security Deposit") in
the amount of NINETEEN THOUSAND SEVEN HUNDRED FIFTY-EIGHT and 38/100 DOLLARS
($19,758.38) to be held by Landlord without interest as security for the
faithful performance and observance by Tenant of the terms, conditions and
provisions of this Sublease, including without limitation the surrender of
possession of the Premises to Landlord as herein provided. Landlord shall not be
required to maintain the Security Deposit in a separate account. It is agreed
that in the event Tenant defaults in respect of any of the terms, provisions and
conditions of this Sublease, including. but not limited to, the payment of Base
Rent and Additional Rent, Landlord may apply or retain the whole or any part of
the Security Deposit to the extent required for the payment of any Base Rent and
Additional Rent or any other sum as to which Tenant is in default or for any sum
which Landlord may expend or may be required to expend by reason of Tenant's
default in respect of any of the terms, covenants and conditions of this
Sublease, including but not limited to, any damages or deficiency in the
reletting of the Premises, whether such damages or deficiency accrue or accrues
before or after summary proceedings or other reentry by Landlord. The Security
Deposit is not to be used or applied by Tenant as a substitute for rent due any
month, but may be so applied by Landlord at any time at Landlord's option. The
use, application or retention of the Security Deposit, or any portion thereof,
by Landlord shall not prevent Landlord from exercising any other right or remedy
provided by this Sublease or by law and shall not operate as a limitation on any
recovery to which Landlord may otherwise be entitled. If Landlord applies or
retains any part of the Security Deposit, Tenant, upon written demand therefor
by Landlord, shall deposit cash with Landlord in such amount so that Landlord
shall have the full deposit on hand at all times during the Term. If Tenant
shall fully and


<PAGE>
                                     - 12 -


faithfully comply with all of the terms, provisions, covenants and conditions of
this Sublease, the balance of the Security Deposit, if any, shall be returned to
Tenant within thirty (30) days after the Expiration Date and after the delivery
of the entire possession of the Premises to Landlord.

         27.      PAYMENT OF THE FIRST MONTH'S BASE RENT. Tenant shall pay to
Landlord the first monthly installment of the applicable Base Rent ($19,758.38)
upon execution of this Sublease.

         28.      SIGNAGE. Any signage contemplated by the Tenant shall be
subject to and in accordance with the requirements set forth in the Prime Lease
(at Section 8.1). Landlord shall have no obligation or responsibility to remove
any signage which exists at the Premises as of the Commencement Date.

         29.      LANDLORD'S REPRESENTATIONS. Landlord hereby represents and
warrants that (i) the Prime Lease is in full force and effect, (ii) the Prime
Lease attached hereto as Exhibit A is the complete Prime Lease and that, except
for the First Amendment to Lease dated June 22, 1998, the Prime Lease has not
been amended or modified; (iii) to the best of Landlord's knowledge, there are
no existing setoffs, defenses or counterclaims against the Prime Landlord with
respect to the payment of rent reserved under the Prime Lease or any performance
of other terms, conditions or covenants of the Prime Lease on the part of the
Tenant under the Prime Lease to be performed; and (iv) there exists no defaults
or breaches of Prime Landlord's or Tenant's obligations under the Prime Lease
nor, to the best of Landlord's knowledge, any event which with the giving of
notice or passage of time, or both, would constitute a default under the Prime
Lease.

         30.      MISCELLANEOUS.

                  (a) This Sublease may not be extended, renewed, terminated, or
otherwise modified except by an instrument in writing signed by the party
against whom enforcement of any such modification is sought.

                  (b) It is understood and agreed that all understandings and
agreements heretofore had between the parties hereto are merged in this
Sublease, which alone fully and completely expresses their agreement, and that
the same is entered into after full investigation, neither party relying upon
any statement, representation or warranty made by the other not embodied in this
Sublease.

                  (c) The paragraph headings appearing herein are for purposes
of convenience only and are not deemed to be a part of this Sublease.

                  (d) The provisions of this Sublease shall be governed by and
construed in accordance with the laws of the Commonwealth of Massachusetts
without regard to principles of conflicts of laws. Landlord and Tenant agree to
submit to jurisdiction in the Commonwealth of Massachusetts with respect to any
dispute under or arising out of this Sublease and agree that any such dispute
shall be brought either in the courts of the


<PAGE>
                                     - 13 -


Commonwealth of Massachusetts or in the applicable federal district court
located in Massachusetts.

                  (e) If any provision of this Sublease or the application
thereof to any person or circumstance shall to any extent be invalid or
unenforceable, the remainder of this Sublease, or the application of such
provision to persons or circumstance other than those as to which it is invalid
or unenforceable, shall not be affected thereby, and each provision of this
Sublease shall be valid and enforced to the fullest extent permitted by law.

                  (f) This Sublease may be executed in counterparts each of
which shall be deemed an original and all of which together shall constitute one
and the same document.

                  (g) This Sublease (or any notice hereof) shall not be
recorded.

                  (h) Landlord and Tenant hereby each waive trail by jury in any
action, proceeding or counterclaim brought by either against the other, on or in
respect of any matter whatsoever arising out of or in any way connected with
this Sublease, the relationship of Landlord and Tenant or Tenant's use or
occupancy of the Premises.

                  (i) This Sublease includes and incorporates all Exhibits
referred to hereby and attached hereto.

<PAGE>
                                     - 14 -


         IN WITNESS WHEREOF, this Agreement of Sublease has been duly executed
as of the day and year first above written.


                                       LANDLORD:
                                       UNISPHERE SOLUTIONS, INC.

                                       By: /s/ Robert T. Curtin
                                          -----------------------------------
                                       Name: Robert T. Curtin
                                            ---------------------------------
                                       Title:   Director of Real Estate
                                            ---------------------------------

                                       TENANT:
                                       SONUS NETWORKS, INC.

                                       By: /s/ S.J. Nill
                                          -----------------------------------
                                       Name: S.J. Nill
                                            ---------------------------------
                                       Title: VP & CFO
                                            ---------------------------------