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                                                                    Exhibit 10.7

                                                       5 Carlisle Road, Westford
                                                                  EXECUTION COPY

                              AGREEMENT OF SUBLEASE

      THIS AGREEMENT OF SUBLEASE ("Sublease") is made as of the 20th day of
October, 2000, by and between Unisphere Networks, Inc., a Delaware corporation
("Landlord"), having an office at One Executive Drive, Chelmsford, MA 01824 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 February 17, 1999, (the "Prime Lease") by
and between Glenborough Fund V, Limited Partnership ("Prime Landlord") and
Redstone Communications, Inc., Prime Landlord leased to Redstone Communications,
Inc. the premises (as hereinafter defined) (the "Premises") which is located
within the building known by the street address 5 Carlisle Road, Westford, MA
01886 (the "Building"), which Premises are more particularly described in the
Prime Lease and include 40,092 rentable square feet consisting of Suites 1E-01
and 1W-01;

      WHEREAS, Unisphere Solutions, Inc. is successor in interest to Redstone
Communications, Inc.;

      WHEREAS, Unisphere Solutions, Inc. changed its name to Unisphere Networks,
Inc. (hereinafter referred to as "Landlord").

      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

            a. The term (the "Term") of this Sublease shall commence on February
1, 2001 (the "Commencement Date"), with said Term to expire at midnight on March
30, 2004 (the "Expiration Date"), unless sooner terminated as hereinafter
provided.

            b. Tenant acknowledges that Landlord is in the process of
constructing a new building into which it intends to relocate its operations
from the Premises. Notwithstanding any other provision of this Sublease to the
contrary, Landlord may deliver the Premises to Tenant at any time from February
1, 2001 to June 30, 2001 upon thirty (30) days notice ("Commencement

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Date Notice") to Tenant, provided that the Commencement Date shall be extended
to such date and the Rent, as hereinafter defined, shall become payable on the
Commencement Date.

            c. Under no circumstances shall Landlord be required to vacate the
Premises if its new building shall not be substantially complete, in Landlord's
sole judgment. In the event that Landlord is unable to deliver the Premises to
Tenant on or before June 30, 2001, then either party may terminate this Sublease
in which case this Sublease shall terminate without further recourse to either
party hereunder and shall be null and void with no further force or effect.

            d. Tenant shall not be required to take delivery of the Premises
unless and until Landlord shall deliver possession of the Premises located at
235 Littleton Road, Westford, Massachusetts.

            e. Tenant shall have the right to have entry and access to the
Premises commencing on the twentieth day after receipt of the Commencement Date
Notice and Tenant shall not be obligated to pay any rent due hereunder but all
other terms and conditions of this Sublease shall be in full force and effect
upon the date of entry into the Premises by Tenant for any purposes.

      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:

--------------------------------------------------------------------------------
Dates                            Annual Rent              Monthly Rent
--------------------------------------------------------------------------------
Commencement Date --             $641,472.00              $53,456.00
March 31, 2001
--------------------------------------------------------------------------------
April 1, 2001 to March 31,       $681,564.00              $56,797.00
2002
--------------------------------------------------------------------------------
April 1, 2002 to March 31,       $701,610.00              $58,467.50
2003
--------------------------------------------------------------------------------
April 1, 2003 to March 30,       $721,656.00              $60,138.00
2004
--------------------------------------------------------------------------------

All such monthly installments shall be paid in advance, on the first (1st) 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, one hundred percent (100%) of all additional rent payments for
Tax Costs and Operating Expenses under Section 6.3 of the Prime Lease (as such
terms are referred to in the Prime Lease). Rental and any other sums due
hereunder not paid by the due date shall bear interest of 18% per annum. 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. Landlord shall furnish to Tenant any
notice or schedule of payments


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provided to Landlord by the Prime Landlord, and thereafter any payments of
Additional Rent shall be made consistent with such notice or schedule without
further demand by Landlord.

            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 as they
related to the Premises in accordance with the terms of the Prime Lease.

            (b) Tenant's obligation to pay Additional Rent hereunder shall be on
account of the period from and after the 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
taxes attributable to leasehold improvements, furniture or equipment 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 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 E and any
rules and regulations promulgated by the Prime Landlord.

            (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 (but only to the extent required by Prime Landlord
pursuant to the Prime Lease), 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 25 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


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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 2.20 of the Prime Lease, "General Office and Light
Manufacturing for Telecommunications Equipment", and for no other purpose. Light
Manufacturing shall be defined as light assembly of networking gear, that is,
installation of components into a case/chassis, including testing.

      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. 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, at the
option of the Prime Landlord, Prime Landlord shall take over all of the right,
title and interest of Landlord under this Sublease and Tenant hereunder shall
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


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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.

      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 4 [Delivery of
Possession], 10 [Services and Utilities], 11.1 [Landlord's Obligations], 12.1
[Landlord's Construction Obligations], 16 [Damage or Destruction] [relating to
any duty to restore the Premises or the Building], 17 [Eminent Domain] [relating
to any duty to restore the Premises or the Building], and 35
[Telecommunications] 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


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may arise, nor (except for any abatement provided to and actually received by
Landlord) 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 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


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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. This subparagraph (d) shall not be applicable to Section 21 of this
Sublease.

            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 the Prime Lease including,
but not limited to, the provisions of Section 14 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.

      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 2.4
(Commencement Date), 2.6 (Expiration Date), 2.8 (Addresses), 2.9 (Listing and
Leasing Agents), 3.4 (Exhibit D -- Work Letter and Drawings), 4 (Delivery of
Possession), 8 (Security Deposit), 12.1 (Landlord's Construction Obligations),
12.2 (Tenant's Construction Obligations), 18 (Assignment and Subletting) (as
between Landlord and Tenant only), 37 (Tenant Improvements), 38 (Assignment and
Subletting) (as between Landlord and Tenant only), 39 (Estoppel Certificates),
and 40 (Damage and Destruction).


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      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 may be given or withheld in Landlord's sole discretion.
Notwithstanding the foregoing, Landlord's consent shall not be unreasonably
withheld with respect to assignments or transfers to Affiliated Entities.
Affiliated Entities shall mean (i) a subsidiary of Tenant, (ii) a corporation or
other entity into or with which Tenant has merged or consolidated or to which
substantially all of Tenant's stock or assets are transferred, or (iii) any
corporation or other entity which controls, is controlled by, or is under common
control with Tenant.

            Notwithstanding the foregoing, provided there is no default
continuing beyond any applicable notice and/or cure period, Landlord also
agrees, upon the written request of Tenant, to not unreasonably withhold its
consent to a sub-sublease of the Premises by Tenant, provided and on the
condition that: (i) such sub-sublease including the subtenant shall be subject
to Prime Landlord's prior approval, which approval shall not be unreasonably
withheld as further described in the Prime Lease; (ii) such subtenant shall be
of a business reputation reasonably acceptable to Landlord; (iii) such subtenant
shall have a financial net worth, determined by generally acceptable accounting
principles, satisfactory to Landlord (and Tenant shall supply financial
information and documentation regarding such financial status to Landlord thirty
(30) days prior to the execution of any new sub-sublease); (iv) the terms of
such new sub-sublease shall be on the same or better terms as this Sublease (or
if any term may be less favorable to


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Landlord, the inclusion of such change in the new sublease shall be subject to
Landlord's sole discretion); (v) the new sub-sublease shall be unconditionally
guaranteed by Tenant, which form of guaranty shall be satisfactory to Landlord;
(vi) Tenant shall reimburse Landlord for its reasonable out-of-pocket expenses,
including reasonable attorney's fees, incurred in connection with such surrender
and sub-sublease and Tenant shall be solely responsible for any leasing
commissions, improvements allowances or other expenses relating to the proposed
sub-sublease; (vii) Tenant shall not be in default hereunder; (viii) Tenant
shall pay to Landlord 50% of all rents received in connection with such
sub-sublease in excess of the rents set forth herein; and (ix) the new
sub-sublease shall be in the form of this Sublease. Any obligations of Tenant to
reimburse Landlord hereunder shall survive the surrender of the Sublease.

            (b) If Tenant requests Landlord's consent to a sublease as set forth
above, Tenant, together with such request for consent, shall provide Landlord
with the name of the proposed transferee and the nature of the business of the
proposed transferee, the term, use, rental rate and all other material terms and
conditions of the proposed transfer, including, without limitation, a copy of
the proposed sub-sublease or other contractual documents and evidence
satisfactory to Landlord that the proposed transferee is financially
responsible.

            With respect to any sub-sublease of fifty (50%) percent or more of
the Premises, Landlord may have the right to recapture the Premises if Tenant
proposes to assign or sublease the Premises to a third party after the one year
anniversary of the Commencement Date. Landlord must exercise (or forego) its
recapture rights within fifteen (15) days of its receipt of Tenant's
sublease/assignment notice. In order to effectuate its recapture rights,
Landlord may terminate, at its election, the sublease with respect to the entire
Premises or with respect to that portion of the Premises described in the
sub-sublease and Tenant shall have no further obligations (except as otherwise
specifically set forth in this Sublease) with respect to that portion of the
Premises which are recaptured (and there shall be a pro rata reduction in Rent
and the Security Deposit based upon the rentable square footage of the
recaptured portion of the Premises relative to the rentable square footage of
Premises prior to recapture), except as specifically set forth herein, and
Tenant shall at its expense erect, if necessary, a demising wall to separate the
remainder of the Premises from that portion of the Premises which is being
recaptured and to take such other steps as shall be necessary to make the
portion of the Premises that is not being recaptured into a self-contained,
lawfully demisable rental unit.

            (c) In the event Landlord is requested to consent to an assignment
of this Sublease or a subletting of the Premises, Tenant shall pay all costs and
expenses in connection with such assignment or subletting, including, without
limitation, any reasonable attorneys' fees of Landlord in connection with its
review of the assignment or subletting documents, any cost of renovating,
altering or decorating the Premises for a new occupancy (if Tenant desires to do
the same) and any leasing brokerage fees payable.


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<PAGE>

            (d) The listing of any name other than that of Tenant, whether on
the doors of the Premises or on the Building directory, or otherwise, shall not
operate to vest any right or interest in this Sublease or in the Premises or be
deemed to be the written consent of Landlord mentioned in this Section, it being
expressly understood that any such listing is a privilege extended by Landlord
revocable at will by written notice to Tenant.

            (e) If this Sublease is assigned, or if the Premises or any part
thereof is sublet or occupied by anybody other than Tenant, Landlord may at any
time and from time to time, collect rent and other charges from the assignees,
subtenants or occupants, and apply the net amount collected to the Base Rent and
other charges herein reserved, but no such assignment, sublease or collection
shall be deemed a waiver of this covenant, or the acceptance of the assignee,
Tenant or occupant as a Landlord, or a release of Tenant from the further
performance of covenants on the part of Tenant herein contained, including
without limitation, Tenant's obligation to pay the Base Rent and other charges,
or a release of the Security Deposit (as hereinafter defined). The consent by
Landlord to an assignment or subletting or occupancy shall not in any way be
construed to relieve Tenant from obtaining the express consent in writing of
Landlord to any further assignment or subletting or occupancy.

            (f) Notwithstanding the immediately preceding sentences, any steps
taken by Landlord or Tenant in this regard shall be subject to all terms and
conditions of the Prime Lease.

            (g) 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.

            (h) 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; Tenant shall
remain directly and primarily liable and any such sublessee or assignee agrees
directly with Landlord by written instrument reasonably satisfactory to Landlord
to be bound by all of the obligations of Tenant.

      19. Insurance.

            (a) Tenant agrees to maintain all insurance coverages specified in
and in accordance with Section 15 of the Prime Lease (including without
limitation commercial general liability and property damage insurance, casualty
insurance and workers' compensation insurance). 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 Building, or the property on which the building is located
(the "Property"), of which Tenant is notified in writing, as their respective
interests may appear. Tenant shall furnish Landlord receipts evidencing payment
of


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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.

      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 Additional Rent 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 all direct, indirect and consequential
damages, costs and expenses incurred by Landlord as a result of such holding
over, including without limitation, any costs and expenses that Prime Landlord
charges to Landlord on account thereof and Tenant agrees to indemnify Landlord
against, and hold Landlord harmless from, any


                                       11
<PAGE>

damage, loss, claim, liability or expenses, including without limitation,
reasonable attorneys' fees arising out of such holding over.

      22. Default.

            (a) In the event that Tenant shall default in the payment of Base
Rent, Additional Rent or any other charge payable hereunder or in the delivery
of any document, instrument or assurance as required hereunder or under the
Prime Lease within two (2) 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,
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.

            (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 twenty (20) 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.

            (c) In addition to any other event of default under this Sublease or
the Prime Lease, it shall be an event of default under this Sublease, without
any notice or cure period, if Tenant shall be in default under that certain
Agreement of Sublease between Landlord and Tenant dated October 20, 2000
relating to space located at 235 Littleton Road, Westford, MA continuing beyond
any applicable notice and/or cure period.

      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, 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.


                                       12
<PAGE>

      Notwithstanding the foregoing, if and only to the extent Prime Landlord
provides Landlord an opportunity under Section 16 of the Prime Lease to
terminate the Prime Lease with respect to the Premises ("Casualty Termination
Option"), Landlord shall within ten (10) business days of receipt from Prime
Landlord of such Casualty Termination Option notify Tenant whether Landlord has
elected to terminate or not terminate the Prime Lease. If Landlord has decided
not to terminate the Prime Lease under Section 16 of the Prime Lease with
respect to the Premises, Tenant may decide, notwithstanding such decision, that
it elects to terminate the Sublease provided (i) there is no default hereunder,
and (ii) Tenant informs Landlord within ten (10) business days of receipt of
such Casualty Termination Option from Landlord that it elects to terminate the
Sublease in which event the Sublease shall terminate on the date set forth in
Section 16 of the Prime Lease. If Tenant fails to satisfy all of the foregoing
conditions, Tenant shall be deemed to have waived any rights set forth herein.
All times set forth herein are of the essence.

      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 Networks, Inc.
                                One Executive Drive
                                Chelmsford, MA 01824
                                      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

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


                                       13
<PAGE>

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 forty-five
(45) 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. (a) Tenant concurrently with the execution of this
Sublease has deposited with Landlord a deposit (the "Security Deposit") in the
amount of One Hundred Six Thousand Nine Hundred Twelve 00/100 Dollars
($106,912.00) 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 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.

            (b) Letter of Credit. Landlord agrees that the Security Deposit may
be provided in the form of an unconditional and irrevocable letter of credit
(the "Letter of Credit"), in a form and by an issuing bank approved by Tenant.
The Letter of Credit shall provide that (a) Landlord can draw upon the Letter of
Credit upon presenting a demand letter to the issuing bank stating that Tenant
is in default under the Sublease after expiration of all applicable notice and
cure


                                       14
<PAGE>

periods or Tenant has failed to provide a replacement letter of credit
acceptable to Landlord as required under the Sublease, and (b) the Letter of
Credit is transferable in whole and not in part by Landlord to Landlord's
transferee, without Tenant's approval, should Landlord transfer or convey its
interest in the Premises. The Letter of Credit shall not expire until a minimum
of thirty (30) days following the expiration of the term of this Sublease.
Landlord shall have the right, from time to time, without prejudice to any other
remedy Landlord may have on account thereof, to draw on the Letter of Credit on
account of, and to apply such drawn amounts to Landlord's damages arising from,
or to cure, any default of Tenant continuing beyond any applicable notice and/or
cure period. If Landlord shall so apply sums drawn on the Letter of Credit,
Tenant shall immediately deliver a replacement Letter of Credit in the same face
amount as the original Letter of Credit before it was drawn upon (or, if
Landlord shall have drawn down only a portion of the Letter of Credit, Tenant
shall deliver a replacement Letter of Credit or an amendment to the original
Letter of Credit such that the aggregate balance available to be drawn down by
Landlord is restored to $106,912). If the term of any Letter of Credit held by
Landlord shall expire prior to the expiration date of this Sublease, and it is
not extended or a new letter of credit for an extended period of time is not
substituted within ten (10) days prior to the expiration date of the Letter of
Credit, then Landlord may make demand for the principal amount of the Letter of
Credit and hold such funds in accordance with this Section 26 until the
expiration date of the term of the Sublease. At any time that Tenant is in
default under the terms of this Sublease continuing beyond any applicable notice
and/or cure period, Landlord may make demand for the principal amount of the
Letter of Credit and hold such funds for the balance of the term of the Sublease
in accordance with this Section 26. All costs and fees associated with issuing
and maintaining the Letter of Credit shall be borne by Tenant.

      27. Payment of the First Month's Base Rent. Tenant shall pay to Landlord
the first monthly installment of the applicable Base Rent upon execution of this
Sublease.

      28. Signage. Any signage contemplated by the Tenant shall be subject the
consent of the Prime Landlord. 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 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.


                                       15
<PAGE>

      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 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.

                      [SIGNATURES APPEAR ON THE NEXT PAGE]


                                       16
<PAGE>

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

                                           LANDLORD:
                                           UNISPHERENETWORKS, INC.


                                           By: /s/ John J. Connolly
                                               ---------------------------
                                           Name: John J. Connolly
                                                 -------------------------
                                           Title: V.P. Finance & CFO
                                                  ------------------------

                                           TENANT:
                                           SONUS NETWORKS, INC.


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


                                       17
<PAGE>

                                    EXHIBIT A
                                   Prime Lease

                                 Attached hereto


                                       18