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

     THIS EMPLOYMENT AGREEMENT (the "Agreement") is entered into by Summa Four,
Inc., a Delaware corporation, with its principal place of business at
Manchester, New Hampshire (the "Company"), and Philip T. Coates (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 June 1, 1997 (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 the Company's Vice President,
     International Sales, currently reporting to the Vice President, World-Wide
     Sales, or in such other more senior or responsible 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 United Kingdom location, or
     such place or places elsewhere internationally or in the continental United
     States as the Board shall reasonably determine and the Employee shall
     agree, for a minimum of twelve months from the Commencement Date. Upon
     completion of the minimum twelve months period from the Commencement Date,


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the Agreement will continue in force subject to the sections 4. Through 4.4 of
this Agreement. The Employee's performance and salary will be reviewed annually
and Employee will be eligible for merit increases in line with the company's
policy. 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.

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 his entire business time, attention
and energies to the business and interests of the Company during the Employment
Period. The Employee agrees to abide by 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.

3.   COMPENSATION AND BENEFITS.

     3.1  SALARY. The Company shall pay the Employee, in bi-weekly installments,
          an annual base salary of US$125,000 (less any applicable deductions
          for taxes, etc.) for the period beginning on the Commencement Date,
          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. The Employee
          shall receive the full benefit of the Company's Tax Equalization
          Program such that the Employee shall not incur liability for taxes on
          wages or other benefits in excess of that which would have been
          incurred if he was located in the United States.

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     3.2  BONUS. For such period ending March 31, 1998, the Employee shall be
          eligible for commissions and bonuses approximating to $US86,000 at
          goal as laid out in the Company Compensation Agreement signed in May
          1997 with subsequent amendments. In order for the Employee to be
          entitled to payment of his commissions and bonus, he must be actively
          employed by the Company on the date they are earned, in accordance
          with the Company's Compensation Plan signed by Employee. For periods
          subsequent to March 31, 1998, the Employee shall be entitled to
          participate in such commission and bonus programs, if any, as may be
          established from time to time by its Board of Directors.

     3.3  GOODS AND SERVICES ALLOWANCE. The Company agrees to pay the Employee
          $1388.11 bi-weekly as an offset for the higher costs of goods and
          services in London. Payments will be less any applicable deductions
          for taxes, etc., commencing with the Employee's move to London and
          concluding with his return to the United States or any change in
          location of assignment.

     3.4  HOUSING ALLOWANCE. The Company agrees to pay the Employee 16,900
          Pounds Sterling quarterly as an offset for the higher cost of housing
          in London. Payments will be less any applicable deductions for taxes,
          etc., commencing with the Employee's move to London and concluding
          with his return to the United States or any change in location of
          assignment. In the event that the Employee's employment is terminated
          by the employer after the twelve month period from the Commencement
          date, the employer will be responsible for the rent and costs
          associated with any unused portion of the Employee's housing
          accommodations.

     3.5  AUTOMOBILE ALLOWANCE. The Company agrees to pay the Employee 861.28
          Pounds Sterling per month to provide for use of an automobile while in
          London. Payments will

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          be less any applicable deductions for taxes, etc., commencing with the
          Employee's move to London and concluding with his return to the United
          States or any change in location of assignment.

     3.6  MOVING EXPENSES. The Company shall cover reasonable moving and travel
          expenses associated with the Employee and his immediate family
          relocating to London. As provided under policy, the Company will also
          cover reasonable moving and travel expenses relating to the Employee's
          return to the United States at the completion of one (1) year from the
          Commencement Date or such earlier date that the Company chooses or
          later date mutually agreed upon.

     3.7  FRINGE BENEFITS. The Employee shall be entitled to participate in all
          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.

     3.8  Reimbursement OF EXPENSES. The Company shall reimburse the Employee
          for all gasoline used by the Employee in one automobile owned or
          leased by him and for reasonable travel, entertainment and other
          expenses incurred, per the Company's Travel and Entertainment Policy,
          or paid by the Employee in connection with, or related to, the
          performance of his duties, responsibilities or services under this
          Agreement, as more fully described in section 12.5.3, 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

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          gasoline will be included in Employee's taxable income except as so
          documented as a business expense.

     3.9  STOCK OPTION. Subject to Board approval, the Employee shall be granted
          an incentive stock option to purchase 18,670 shares (5,000 of which
          are to be re-issued shares to replace the 5,000 shares the employee
          received on April 1, 1996) of the Company's common stock under its
          1993 Stock Incentive Plan (the "Plan"). The option price shall be the
          closing price on the first Monday in May, 1997. The option to purchase
          18,670 shares shall vest over three years at 33 1/3% per year. If, in
          the future, the Board votes to shorten the vesting schedule for
          Officers generally, then any of the Employee's unvested shares would
          fall under the new, more favorable vesting schedule. Stock options
          granted may be exercised by the Employee, to the extent vested in
          accordance with the terms of the Plan. If, after completing six months
          of service, Employee's employment is involuntarily terminated by the
          Company for any reason other than "Cause", any unvested shares, which
          would have otherwise vested within 12 months of such involuntary
          termination, shall vest immediately.

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 his 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 his actual employment by the Company. For
          purposes of this Agreement, "cause" for termination shall be deemed to
          exist upon (a) alcohol or drug abuse affecting the

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          performance of Employee's duties, theft, embezzlement, fraud,
          absenteeism, 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 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 his employment
          because of death or disability occurs, as well as any benefits due
          Employee under the benefits programs, e.g., life insurance. 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 current base
          salary and insurance benefits, 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
                
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          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 other than cause within
          one year after a change in control, or the Employee experiences a
          substantial diminution in the nature or status of his responsibilities
          from those set forth in his job description, and the Employee chooses
          to terminate employment, the Company shall continue paying the
          Employee's current base salary and benefits, after the Employee's
          termination of employment, for up to twelve (12) months (the
          "Severance 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.   REIMBURSEMENT TO THE COMPANY. In the event that the Employee terminates his
employment with the Company other than for illness or injury within one (1) year
from the Commencement date as defined in paragraph one (1) of this Agreement,
the Employee agrees to reimburse the Company for the full amount of the Goods
and Services Allowance (as specified under provision 3.3) and the full amount
the Company paid for the Employee's moving and travel expenses associated with
his move to London (as specified under provision 3.6).

6.   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 knowingly 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 customer of Excel (including, without
          limitation, Boston Technology, Glenayre or Access Line); 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, 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 6 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 too broad a
     geographic area, it shall be

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     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 6 are necessary for the
     protection of the business and goodwill of the Company and are considered
     by the Employee reasonable for such purpose. The Employee agrees that any
     breach of this Section 6 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.

7.   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 7.

8.   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.

9.   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.

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

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

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12.  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.

13.  MISCELLANEOUS.

     13.1 No delay or omission by the Company in exercising any right under this
     Agreement shall operate as a waiver of that or any 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.

     13.2 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.

     13.3 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.

     13.4 The Employee shall be entitled to indemnification as an Officer of the
     Company under the Company's by-laws and charter.


IN WITNESS WHEREOF, the parties hereto have executed this Agreement as of this
fourteenth day of February, 1998.


SUMMA FOUR, INC.                      PHILIP T. COATES     

By: /s/ Todd P. Hausmann              By: /s/ P. Coates
   -------------------------------       ---------------------------------------

Title: V.P. Worldwide Sales           Title: Vice President, International Sales
       ---------------------------          ------------------------------------


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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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                                   ADDENDUM A

     Summa Four agrees to waive the recovery of any overpayment of the Goods and
Services Allowance in 3.3.3, which may have been paid to Employee between June
1, 1997 and December 4, 1997 following Employee's relocation from Singapore to
London.

SUMMA FOUR, INC.                      PHILIP T. COATES     

By: /s/ Lacey Achausmann              By: /s/ P. Coates
   -------------------------------       ---------------------------------------

Title: Director, Human Resources      Title: Vice President, International Sales
       ---------------------------          ------------------------------------

Date 3/11/98                          Date 3.28.98  
     -----------------------------         -------------------------------------

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