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                                                                  Exhibit 10.28


                              EMPLOYMENT AGREEMENT
                              --------------------

         THIS EMPLOYMENT AGREEMENT (the "Agreement"), made this 22nd day of
July, 1996, is entered into by Summa Four, Inc., a New Hampshire corporation
with its principal place of business at Manchester, New Hampshire (the
"Company"), and Theresa Pratt Wang (the "Employee").

         The Company desires to employ the Employee, and the Employee desires to
be employed by the Company. In consideration of the mutual covenants and
promises contained herein, and other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged by the parties hereto,
the parties agree as follows:

         1. EMPLOYMENT AT WILL. The Company hereby agrees to employ the Employee
as an employee at will, and the Employee hereby accepts such employment with the
Company, upon the terms set forth in this Agreement, for the period commencing
on July 1, 1996 (the "Commencement Date") and ending on the date the Employee's
employment is terminated in accordance with the provisions of Section 4 (such
period, the "Employment Period").

         2. TITLE; CAPACITY. The Employee shall serve as Vice President,
Engineering or in such other position as the Company or its Board of Directors
(the "Board") may determine from time to time. The Employee shall be based at
the Company's headquarters in Manchester, New Hampshire, or such place or places
in the continental United States as the Board shall reasonably determine and the
Employee shall agree. The Employee shall be subject to the supervision of, and
shall have such authority as is delegated to him by, the Board or such officer
of the Company as may be designated by the Board.


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         The Employee hereby accepts such employment and agrees to undertake the
duties and responsibilities inherent in such position and such other duties and
responsibilities as the Board or its designee shall from time to time reasonably
assign to him. The Employee agrees to devote her entire business time, attention
and energies to the business and interests of the Company during the Employment
Period. The Employee agrees to abide by the rules, regulations, instructions,
personnel practices and policies of the Company and any changes therein which
may be adopted from time to time by the Company. The Employee acknowledges
receipt of copies of all such rules and policies committed to writing as of the
date of this Agreement.

         3.       Compensation and Benefits.
                  -------------------------

                  3.1 SALARY. The Company shall pay the Employee, in bi-weekly
installments, an annual base salary of $150,000 (less applicable state and
federal taxes) for the period commencing on the Commencement Date and ending
March 31, 1997, subject to adjustment thereafter as determined annually by the
Board at its first regular meeting following its receipt of the Company's
audited financial results for its preceding fiscal year.

                  3.2 BONUS. For such period ending March 31, 1997, the Employee
shall be eligible for a bonus of up to 40% of her current base salary based on
the achievement of certain goals approved by the Company's Compensation
Committee. Half of the bonus shall be based on the satisfaction of personal
objectives and the other half shall be based on the satisfaction of corporate
objectives for the Company's fiscal year then ending, as determined by the
Company's management and approved by its Board of

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Directors. If either the Employee's personal objectives or corporate objectives
are satisfied, the Employee shall receive that portion of her bonus related to
the satisfaction of either of those objectives without regard to whether the
other is satisfied. In order for the Employee to be entitled to payment of her
bonus, he must be actively employed on the date the bonus is paid. For periods
thereafter, the Employee shall be entitled to participate in such bonus
programs, if any, as may be established from time to time by its Board of
Directors.

                  3.3 FRINGE BENEFITS. The Employee shall be entitled to
participate in all bonus and benefit programs that the Company establishes and
makes available to its employees, if any, to the extent that Employee's
position, tenure, salary, age, health and other qualifications make him eligible
to participate, and to vacations in accordance with the Company's vacation
policy.

                  3.4 REIMBURSEMENT OF EXPENSES. The Company shall reimburse the
Employee for all gasoline used by the Employee in one automobile owned or leased
by her and for reasonable travel, entertainment and other expenses incurred or
paid by the Employee in connection with, or related to, the performance of her
duties, responsibilities or services under this Agreement, upon presentation by
the Employee of documentation, expense statements, vouchers and/or such other
supporting information as the Company may request, PROVIDED, HOWEVER, that (i)
the amount available for such travel, entertainment and other expenses may be
fixed in advance by senior management or the Board and (ii) amounts so
reimbursed for gasoline will be included in Employee's taxable income except as
so documented as a business expense.

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                  3.5 STOCK OPTION. At the Board's July 19, 1996 meeting, the
Employee shall be granted an incentive stock option to purchase 7,500 shares of
the Company's common stock under its 1993 Stock Incentive Plan (the "Plan").
This new grant shall be in addition to all pre-existing grants. The new option
to purchase 7,500 shares shall vest one-third on July 19, 1997, one-third on
July 19, 1998 and one-third on July 19, 1999. Stock options granted may be
exercised by the Employee, to the extent vested, in accordance with the terms of
the Plan.

         4.       EMPLOYMENT TERMINATION WITHOUT CAUSE. The Employee shall have
the status of an employee at will. The Company may terminate the Employee's
employment at any time without cause. The Employee has no obligation to remain
employed by the Company and may terminate her employment at any time.

                  4.1 TERMINATION FOR CAUSE. In the event the Employee's
employment is terminated for cause the Company shall pay to the Employee the
compensation and benefits otherwise payable to him under Section 3 through the
last day of her actual employment by the Company. For purposes of this
Agreement, "cause" for termination shall be deemed to exist upon (a) a good
faith finding by the Company of failure of the Employee to perform her assigned
duties for the Company, dishonesty, gross negligence or misconduct, or (b) the
conviction of the Employee of, or the entry of a pleading of guilty or nolo
contendere by the Employee to, any crime involving moral turpitude or any
felony.

                  4.2 TERMINATION FOR DEATH OR DISABILITY. If the Employee's
employment is terminated by death or because of disability, the Company shall
pay to the estate of

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the Employee or to the Employee, as the case may be, the compensation which
would otherwise be payable to the Employee up to the end of the month in which
the termination of her employment because of death or disability occurs. For
purposes of this Agreement, the term "disability" shall mean the inability of
the Employee, due to a physical or mental disability, for a period of 90 days,
whether or not consecutive, during any 360-day period to perform the services
contemplated under this Agreement. A determination of disability shall be made
by a physician satisfactory to both the Employee and the Company, PROVIDED THAT
if the Employee and the Company do not agree on a physician, the Employee and
the Company shall each select a physician and these two together shall select a
third physician, whose determination as to disability shall be binding on all
parties.

                  4.3 TERMINATION BY THE COMPANY OTHER THAN FOR CAUSE. If the
Employee's employment is terminated by the Company for any reason other than for
cause, the Company shall continue paying the Employee's base salary, after the
Employee's termination of employment, for up to six (6) months (the "Severance
Benefit"). Payment of the Severance Benefit shall cease upon the earlier of (a)
the date six (6) months after the Employee's date of termination, or (b) the
date the Employee begins employment with another employer (or is re-employed by
the Company).

                  4.4 TERMINATION FOLLOWING CHANGE IN CONTROL. If the Employee's
employment is terminated by the Company for any reason within one year after a
change in control, the Company shall continue paying the Employee's base salary,
after the Employee's termination of employment, for up to twelve (12) months
(the "Severance

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Benefit"). Payment of the Severance Benefit shall cease upon the earlier of (a)
the first anniversary of the Employee's date of termination, or (b) the date the
Employee begins employment with another employer (or is re-employed by the
Company). For the purposes of this paragraph, a change in control is defined as
provided for in Attachment "A".

         5.       Non-Compete.
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                  (a)      During the Employment Period and for a period of
twelve (12) months after the Employee's termination of employment for any
reason, the Employee will not directly or indirectly:

                  (i)      as an individual proprietor, partner, stockholder,
officer, employee, director, joint venturer, investor, lender, consultant or in
any other capacity whatsoever (other than as the holder of not more than one
percent (1%) of the total outstanding stock of a publicly held company), render
any services to any business engaged in the design, manufacture or sale of
programmable switches (including, without limitation, Excel), or to any
non-legacy switch manufacturer (including, without limitation, Sattel) or any
major original equipment manufacturer customers of Excel (including, without
limitation, Boston Technology, Glenayre or AccessLine); or

                  (ii)     recruit, solicit or induce, or attempt to induce, any
employee or employees of the Company to terminate their employment with, or
otherwise cease their relationship with, the Company; or

                  (iii)    solicit, divert or take away, or attempt to divert or
to take away, the business or patronage of any of the clients, customers or
accounts, or prospective clients,

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customers or accounts, of the Company which were contacted, solicited or served
by the Employee while employed by the Company.

                  (b) If any restriction set forth in this Section 5 is found by
any court of competent jurisdiction to be unenforceable because it extends for
too long a period of time or over too great a range of activities or in too
broad a geographic area, it shall be interpreted to extend only over the maximum
period of time, range of activities or geographic area as to which it may be
enforceable.

                  (c) The restrictions contained in this Section 5 are necessary
for the protection of the business and goodwill of the Company and are
considered by the Employee to be reasonable for such purpose. The Employee
agrees that any breach of this Section 5 will cause the Company substantial and
irrevocable damage and therefore, in the event of any such breach, in addition
to such other remedies which may be available, the Company shall have the right
to seek specific performance and injunctive relief.

         6.       NOTICES. All notices required or permitted under this
Agreement shall be in writing and shall be deemed effective upon personal
delivery or upon deposit in the United States Post Office, by registered or
certified mail, postage prepaid, addressed to the other party at the address
shown above, or at such other address or addresses as either party shall
designate to the other in accordance with this Section 6.

         7.       PRONOUNS. Whenever the context may require, any pronouns used
in this Agreement shall include the corresponding masculine, feminine or neuter
forms, and the singular forms of nouns and pronouns shall include the plural,
and vice versa.

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         8.       ENTIRE AGREEMENT. This Agreement constitutes the entire
agreement between the parties and supersedes all prior agreements and
understandings, whether written or oral, relating to the subject matter of this
Agreement.

         9.       AMENDMENT. This Agreement may be amended or modified only by a
written instrument executed by both the Company and the Employee.

         10.      GOVERNING LAW. This Agreement shall be construed, interpreted
and enforced in accordance with the laws of the State of New Hampshire.

         11.      SUCCESSORS AND ASSIGNS. This Agreement shall be binding upon
and inure to the benefit of both parties and their respective successors and
assigns, including any corporation with which or into which the Company may be
merged or which may succeed to its assets or business, provided, however, that
the obligations of the Employee are personal and shall not be assigned by him.

         12.      Miscellaneous.
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                  12.1 No delay or omission by the Company in exercising any
right under this Agreement shall operate as a waiver of that or any other right.
A waiver or consent given by the Company on any one occasion shall be effective
only in that instance and shall not be construed as a bar or waiver of any right
on any other occasion.

                  12.2 The captions of the sections of this Agreement are for
convenience of reference only and in no way define, limit or affect the scope or
substance of any section of this Agreement.

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                  12.3 In case any provision of this Agreement shall be invalid,
illegal or otherwise unenforceable, the validity, legality and enforceability of
the remaining provisions shall in no way be affected or impaired thereby.



         IN WITNESS WHEREOF, the parties hereto have executed this Agreement as
of the day and year set forth above.

                                    SUMMA FOUR, INC.



                                    By:  /s/ Barry Gorsun
                                       -----------------------------------------
                                    Title:  Chairman
                                          --------------------------------------

                                    THERESA PRATT WANG

                                       /s/ Theresa Pratt Wang
                                    --------------------------------------------

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                                 ATTACHMENT "A"

         For the purposes of this Agreement, a change of control shall be deemed
to have occurred if at any shareholders meeting fifty percent (50%) of the total
shares voting of the shares of the Company's common stock outstanding are voted
either directly or by proxy for a person or persons other than those nominated
by the Company's Board of Directors or if the individuals who, as of the date
hereof, constitute the Board of Directors of the Company, cease for any reason
to constitute at least a majority of the Board, provided that any person
becoming a director subsequent to the date hereof whose election or nomination
for election by the Company's shareholders was approved of by a vote of at least
a majority of the Directors then comprising the Board, shall be, for the
purposes of this Agreement, considered as though such person was a member of the
Board as of the date hereof.




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