<PAGE>


                                                                   Exhibit 10.15

                             SECURED PROMISSORY NOTE

$110,250.00                                              Westford, Massachusetts
                                                         As of September 1, 1999

      FOR VALUE RECEIVED, Stephen Nill (the "Maker"), promises to pay to Sonus
Networks, Inc. (the "Company"), or order, at the offices of the Company or at
such other place as the holder of this Note may designate, on or before the Due
Date (as defined below), the principal sum of $110,250.00. Interest shall accrue
on the outstanding principal amount from the date hereof through and including
the date on which such principal amount is paid in full, computed on the basis
of a year of 365 or 366 days, as the case may be, at a rate per annum of 8%,
compounded annually and payable on the maturity hereof.

      The "Due Date" shall be the earlier of (a) five (5) years from the date
hereof, and (b) 180 days after the date on which a portion of the Collateral (as
defined below) is eligible for sale to the public under the Securities Act of
1933, as amended, free of any restrictions on transfer under securities laws or
agreements between the Maker and the Company or any of the Company's
underwriters.

      Payment of this Note is secured by a security interest in certain property
of the Maker (the "Collateral") pursuant to a pledge agreement of even date
herewith between the Maker and the Company (the "Pledge Agreement").

      This Note shall become immediately due and payable upon demand by the
Company to the Maker upon the occurrence at any time of any Event of Default
under and as defined in the Pledge Agreement.

      Upon the occurrence of an Event of Default, the holder shall have then, or
at any time thereafter, all of the rights and remedies afforded under the Pledge
Agreement and by the Uniform Commercial Code as from time to time in effect in
the Commonwealth of Massachusetts or afforded by other applicable law.

      Notwithstanding anything herein to the contrary, in the event the Company
exercises its repurchase rights with respect to the Collateral pursuant to the
terms of that certain Stock Repurchase Agreement, dated as of the date hereof
between the Company and the Maker, then (a) all of the unpaid principal and
interest hereunder shall be immediately due and payable automatically and
without presentment, demand, protest or other notice of any kind, all of which
are hereby expressly waived by the Maker, and (b) the Company shall set off any
amounts due from Maker to the Company hereunder against any amounts payable by
the Company to the Maker as a result of the repurchase of the Collateral.

      Every amount overdue under this Note shall bear interest from and after
the date on which such amount first became overdue at an annual rate which is
two (2) percentage points above the rate per year specified in the first
paragraph of this Note. Such interest on overdue amounts under this Note shall
be payable on demand and shall accrue and be compounded monthly until the
obligation of the Maker with respect to the payment of such interest has been
discharged (whether before or after judgment).
<PAGE>

      In no event shall any interest charged, collected or reserved under this
Note exceed the maximum rate then permitted by applicable law and if any such
payment is paid by the Maker, then such excess sum shall be credited by the
holder as a payment of principal.

      All payments by the Maker under this Note shall be made without set-off or
counterclaim and be free and clear and without any deduction or withholding for
any taxes or fees of any nature whatever, unless the obligation to make such
deduction or withholding is imposed by law.

      Whenever any amount is paid under this Note, all or part of the amount
paid may be applied to principal, premium or interest in such order and manner
as shall be determined by the holder in its discretion.

      No reference in this Note to the Pledge Agreement or any guaranty shall
impair the obligation of the Maker, which is absolute and unconditional to pay
all amounts under this Note strictly in accordance with the terms of this Note.

      The Maker agrees to pay on demand all costs of collection, including
reasonable attorneys' fees, incurred by the holder in enforcing the obligations
of the Maker under this Note.

      No delay or omission on the part of the holder in exercising any right
under this Note or the Pledge Agreement shall operate as a waiver of such right
or of any other right of such holder, nor shall any delay, omission or waiver on
any one occasion be deemed a bar to or waiver of the same or any other right on
any future occasion. The Maker and every indorser or guarantor of this Note
regardless of the time, order or place of signing waives presentment, demand,
protest and notices of every kind and assents to any extension or postponement
of the time of payment or any other indulgence, to any substitution, exchange or
release of collateral, and to the addition or release of any other party or
person primarily or secondarily liable.

      This Note may be prepaid in whole or in part at any time or from time to
time in the sole discretion of the Maker. Any such prepayment shall be without
premium or penalty.

      None of the terms or provisions of this Note may be excluded, modified or
amended except by a written instrument duly executed on behalf of the holder
expressly referring to this Note and setting forth the provision so excluded,
modified or amended.

                  [remainder of page intentionally left blank]


                                       -2-
<PAGE>

All rights and obligations hereunder shall be governed by the laws of the
Commonwealth of Massachusetts and this Note is executed as an instrument under
seal.

                                                   /s/ Stephen Nill
                                                   ----------------
                                                     Stephen Nill


IN THE PRESENCE OF:

/s/ Victoria B. Lankiewicz
--------------------------

<PAGE>

                                PLEDGE AGREEMENT

This is a pledge agreement made as of the 1st day of September, 1999, between
Stephen Nill ("Pledgor"), and Sonus Networks, Inc., a Delaware corporation with
its principal office at 5 Carlisle Road, Westford, MA 01886 ("Pledgee").

      1. Pledge of Collateral. In order to secure the due and punctual payment
and performance of the Obligations (as defined below), Pledgor hereby pledges,
assigns and grants to Pledgee a continuing security interest and lien on 225,000
shares of Common Stock of the Pledgee ("Common Stock") held by the Pledgor and
all income therefrom, increases therein and proceeds thereof, the certificates
for which Pledgor has delivered to Pledgee together with a stock transfer power
executed in blank (the "Collateral").

      2. Obligations Secured. The security interest in the Collateral granted
hereby secures payment and performance of those obligations described in that
certain promissory note from Pledgor to Pledgee of even date herewith in the
principal amount of $110,250.00 (the "Note"), together with all interest and
other amounts from time to time payable thereunder, with respect to such debt
(the "Obligations").

      3. Pledgee's Rights and Duties with Respect to the Collateral. Pledgee's
only duty with respect to the Collateral shall be to exercise reasonable care to
secure the safe custody thereof. Pledgee shall have the right but not the
obligation to (a) demand, sue for, receive and collect all money or money
damages payable on account of any Collateral, (b) protect, preserve or assert
any other rights of Pledgor or take any other action with respect to the
Collateral, and (c) pay any taxes, liens, assessments, insurance premiums or
other charges pertaining to the Collateral. Any expenses incurred by Pledgee
under the preceding sentence shall be paid by Pledgor upon demand, become part
of the Obligations secured by the Collateral and bear interest at the rate
provided in the Note until paid. Pledgee shall be relieved of all responsibility
for the Collateral upon surrendering it to Pledgor.

      4. Pledgor's Warranties and Indemnity. Pledgor represents, warrants and
covenants (a) that it is and will be the lawful owner of the Collateral, (b)
that the Collateral is and will remain free and clear of all liens, encumbrances
and security interests other than the security interest granted by Pledgor
hereunder, and (c) that Pledgor has the sole right and lawful authority to
pledge the Collateral and otherwise to comply with the provisions hereof. In the
event that any adverse claim is asserted in respect of the Collateral or any
portion thereof, except such as may result from an act of Pledgee not authorized
hereunder, Pledgor promises and agrees to indemnify Pledgee and hold Pledgee
harmless from and against any losses, liabilities, damages, expenses, costs and
reasonable counsel fees incurred by Pledgee in exercising any right, power or
remedy of Pledgee hereunder or defending, protecting or enforcing the security
interests created hereunder. Any such loss, liability or expense so incurred
shall be paid by Pledgor upon demand, become part of the Obligations secured by
the Collateral and bear interest at the rate provided in the Note until paid.

<PAGE>

      5. Covenants of Pledgor. The Pledgor shall not sell, assign or otherwise
transfer any of his right, title or interest in or to any of the Collateral
without the prior written consent of the Company, except for (a) the pledge of
the Collateral to the Company pursuant hereto, (b) transfers of the Collateral
to the Company pursuant to the Stock Repurchase Agreement, dated as of the date
hereof (the "Stock Repurchase Agreement"), between the Pledgor and the Pledgee,
and (c) transfers of the Collateral pursuant to Section 5 of the Stock
Repurchase Agreement, provided that (i) any transfer of Collateral pursuant to
clause (c) above shall be subject to the Company's continuing security interest
and lien on such Collateral hereunder and (ii) any transferee of Collateral
transferred pursuant to clause (c) above shall have executed and delivered to
the Company a written acknowledgement in form and substance reasonably
satisfactory to the Company to the effect that the transfer of such Collateral
is subject to the Company's continuing security interest in and lien on such
Collateral hereunder.

      6. Voting of Collateral. While Pledgor is not in default hereunder,
Pledgee shall have none of the rights conferred upon stockholders of the Common
Stock of the Pledgor, except as otherwise provided herein.

      7. Dividends and Other Distributions. While Pledgor is not in default
hereunder, Pledgor may receive cash dividends and other distributions payable
with respect to the Collateral. Pledgor shall cause all non-cash dividends and
distributions with respect to the Collateral to be distributed directly to
Pledgee, to be held by Pledgee as additional Collateral, and if any such
distribution is made to Pledgor he shall receive such distribution in trust for
Pledgee and shall immediately transfer it to Pledgee.

      8. Pledgor's Default. Pledgor shall be in default hereunder upon the
occurrence of any of the following events (each, an "Event of Default"):

            (a) If Pledgor fails to pay or perform any of the Obligations when
such payment or performance is due;

            (b) If any lien, encumbrance or adverse claim of any nature
whatsoever is asserted with respect to any Collateral or if Pledgor attempts to
transfer the Collateral in contravention of the terms hereof; provided, however,
that the occurrence of an event described in this Section 7(b) shall not
constitute an Event of Default if it relates to the Maker's sale of the
Collateral on margin for the purpose of satisfying outstanding Obligations;

            (c) If Pledgor fails to fulfill in any material respect any
obligation or covenant hereunder;

            (d) If the liquidation, termination of existence, dissolution,
insolvency or business failure of the Pledgor occurs, or if a receiver or
custodian for the Pledgor or any part of its property is appointed if such
appointment is not terminated or dismissed within thirty (30) days;


                                      -2-
<PAGE>

            (e) If any proceedings under the United States Bankruptcy Code or
any other federal or state bankruptcy, reorganization, receivership, insolvency
or other similar law affecting the rights of creditors generally, which
proceeding is not dismissed within thirty (30) days of filing, are instituted
against the Pledgor; or

            (f) If any proceedings under the United States Bankruptcy Code or
any other federal or state bankruptcy, reorganization, receivership, insolvency
or other similar law affecting the rights of creditors generally are instituted
by the Pledgor or if the Pledgor makes a composition or an assignment or trust
mortgage for the benefit of creditors.

      9. Pledgee's Rights upon Default. Upon the occurrence and during the
continuance of an Event of Default, the Pledgee shall thereafter have the
following rights and remedies (to the extent permitted by applicable law) in
addition to the rights and remedies of a secured party under the Uniform
Commercial Code of Massachusetts, all such rights and remedies being cumulative,
not exclusive, and enforceable alternatively, successively or concurrently:

            (i) If the Pledgee so elects and gives written notice of such
election to the Pledgor, the Pledgee may vote any or all of the Collateral
possessing voting rights (whether or not the same shall have been transferred
into its name or the name of its nominee or nominees) and give all consents,
waivers and ratifications in respect of the Collateral and otherwise act with
respect thereto as though it were the outright owner thereof, the Pledgor hereby
irrevocably constituting and appointing the Pledgee the proxy and
attorney-in-fact of the Pledgor, with full power of substitution, to do so;

            (ii) The Pledgee may demand, sue for, collect or make any compromise
or settlement in respect of any Collateral held by it hereunder that it deems
suitable;

            (iii) After ten (10) days' written notice to the Pledgor, the
Pledgee may sell, resell, assign and deliver, or otherwise dispose of any or all
of the Collateral, for cash and/or credit and upon such terms at such place or
places and at such time or times and to such persons, firms, companies or
corporations as the Pledgee shall approve, all without demand for performance by
the Pledgor or advertisement or any further notice whatsoever except such as may
be required by law; and

            (iv) The Pledgee may at any time, at its option, cause all or any
part of the Collateral held by it to be transferred into its name or the name of
its nominee or nominees, receive any income thereon and hold such income as
additional collateral or apply it to the obligations hereunder.

            Notwithstanding anything herein to the contrary, if an Event of
Default shall occur, then the Pledgee may declare all of the unpaid principal
and interest under the Note to be immediately due and payable, all in accordance
with the terms of the Note.


                                      -3-
<PAGE>

      10. Application of Collateral. In the event of a sale of the Collateral,
the proceeds of the sale shall first be applied to the payment of the expenses
of the sale, including broker's commissions, counsel fees, any taxes or other
charges imposed by law upon the Collateral or the transfer thereof and all other
charges paid or incurred by Pledgee pertaining to the sale; and, second, to
satisfy outstanding Obligations, in the order in which Pledgee elects in its
sole discretion; and, third, the surplus (if any) shall be paid to Pledgor. In
the event the Pledgee takes title to the Collateral in accordance with the terms
hereof, the Pledgee shall issue the Pledgor a certificate representing shares of
Common Stock equal in value to the excess of (a) the fair market value of the
Collateral (as determined by the Board of Directors of the Pledgee) over (b) the
amount of outstanding Obligations as of the date Pledgee takes title.

      11. Termination. This Agreement shall terminate in its entirety upon full
payment of the Obligations to Pledgee, and Pledgee shall return any remaining
Collateral to Pledgor within three (3) business days after such termination.

      12. Notices. All notices made or required to be made hereunder shall be
sent by United States certified or registered mail, with postage prepaid, or
delivered by telecopy or by hand to Pledgee or to Pledgor at the addresses first
above written. Notice by mail shall be deemed to have been made on the date when
the Notice is deposited in the mail.

      13. Heirs. Successors. Etc. This Pledge Agreement and all of its terms and
provisions shall benefit and bind the heirs, successors, assigns, transferees,
executors and administrators of each of the parties hereto.

      14. Pledgee's Forbearance. Any forbearance, failure to delay by Pledgee in
exercising any right, power or remedy hereunder shall not be deemed a waiver of
such right, power or remedy.

      EXECUTED under seal as of the date first above written.

PLEDGOR:                             PLEDGEE:

                                     SONUS NETWORKS, INC.


/s/ Stephen Nill                     By:
---------------------                   ------------------------
Stephen Nill                         Title:


                                      -4-