<PAGE>   1
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

   
                                  FORM 10-K/A
                          Amendment No. 2 to Form 10-K
    

   
[X]  Annual Report Pursuant To Section 13 or 15(d) of The Securities Exchange
     Act of 1934
    

For the fiscal year ended March 31, 1995

                         Commission file number 0-22210


   
                                SUMMA FOUR, INC.
             ------------------------------------------------------
             (Exact name of registrant as specified in its charter)
    


   
           Delaware                                   02-0329497
-------------------------------                 ----------------------
(State or other jurisdiction of                   (I.R.S. employer
incorporation or organization)                  identification number)
    


   
                               25 Sundial Avenue
                    ---------------------------------------
                        Manchester, New Hampshire 03103
                    (Address of principal executive office)
    


   
                                 (603) 625-4050
               --------------------------------------------------
               Registrant's telephone number, including area code
    


Securities registered pursuant to Section 12(b) of the Act: None

Securities registered pursuant to Section 12(g) of the Act:  Common Stock,
                                                             $.01 Par Value
                                                            ---------------
                                                            (Title of Class)
 
Indicate by check mark whether the registrant (1) has filed all reports
required to be filed by Section 13 or 15 (d) of the Securities Exchange Act of
1934 during the preceding 12 months (or for such shorter period that the
registrant was required to file such reports), and (2) has been subject to such
filing requirements for the past 90 days.

Yes  X     No
   ----      ---- 

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405
of Regulation S-K is not contained herein, and will not be contained, to the
best of registrant's knowledge, in definitive proxy or information statements
incorporated by reference in Part III of this Form 10-K or any amendment to
this Form 10-K. (X).
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The aggregate market value of the voting stock held by non-affiliates of the
registrant was approximately $150,438,000 as of April 28, 1995. There were
6,354,537 shares of Common Stock outstanding as of May 31, 1995.

                      DOCUMENTS INCORPORATED BY REFERENCE

Portions of the definitive proxy statement (the "Definitive Proxy Statement")
to be filed with the Securities and Exchange Commission relative to the
Company's 1995 annual meeting of stockholders are incorporated by reference in
Part III.

   
This Amendment No. 2 on Form 10-K/A to the Registrant's Annual Report on Form
10-K for the fiscal year ended March 31, 1995 (the "Report") is being filed to
amend and restate the Exhibit Index to the Report. Such Exhibit Index is hereby
amended and restated in its entirety. The actual exhibits previously filed with
the Securities and Exchange Commission with the Report, or incorporated therein
by reference, are not being amended. New exhibits listed are being filed
herewith.
    

                                 Exhibit Index

(3)  Listing of Exhibits

          Exhibit No.                        Description
          -----------                        -----------

   
              3.1         -  Amended and Restated Certificate of Incorporation
                             of the Registrant. (1)

              3.2         -  Amended and Restated By-laws of the Registrant. (1)

              4.1         -  Specimen Certificate representing the Registrant's
                             Common Stock. (1)

              4.2         -  Rights Agreement, dated February 22, 1995, by and 
                             among the Registrant and State Street Bank and 
                             Trust Company. (5)
             
             10.1         -  Lease Agreement, dated July 18, 1990, by and
                             between the Registrant and Northern Manchester 
                             Trust. (1)

             10.2         -  Lease Agreement, as amended, dated December 21,
                             1992, by and between the Registrant and 1100
                             Corporation. (1)
    
<PAGE>   3
   

10.3   --  Lease Agreement, dated February 15, 1993, by and between the
           Registrant and Atrium Executive Center, Inc. (1)

10.4   --  Registration Agreement, dated July 25, 1984, as amended, by and
           among the Registrant and certain investors. (Originally filed as
           Exhibit 10.5)(1)

10.5   --  Purchase Agreement, dated July 25, 1984, by and among the Registrant
           and certain investors. (Originally filed as Exhibit 10.6)(1)

10.6   --  +1994 Executive Incentive Bonus Plan. (Originally filed as 
           Exhibit 10.7)(1)

10.7   --  +1993 Stock Incentive Plan, as amended. (Originally filed as 
           Exhibit 10.8)(1)

10.8   --  +1993 Employee Stock Purchase Plan. (Originally filed as 
           Exhibit 10.9)(1)

10.9   --  +1993 Director Stock Option Plan. (Originally filed as 
           Exhibit 10.10)(1)

10.10  --  +Stock Option Plan of August 1, 1992. (Originally filed as 
           Exhibit 10.11)(1)

10.11  --  +Incentive Stock Option Plan of January 1, 1985. (Originally filed 
           as Exhibit 10.12)(1)

10.12  --  +Stock Option Agreement, dated July 23, 1987, by and between the
           Registrant and Mr. William M. Scranton. (Originally filed as 
           Exhibit 10.13)(1)

10.13  --  +Non-Qualified Employee Option Agreement, dated July 23, 1987, by
           and between the Registrant and Mr. Barry R. Gorsun. (Originally 
           filed as Exhibit 10.14)(1)

10.14  --  +Stock Option Agreement, dated July 1, 1984, by and between the
           Registrant and Mr. Robert A. Degan. (Originally filed as 
           Exhibit 10.15)(1)

10.15  --  +Employment Agreement, dated April 3, 1993, by and between the
           Registrant and Thomas A. St. German. (Originally filed as 
           Exhibit 10.16)(1)

10.16  --  +Agreement, dated April 30, 1991, by and between the Registrant and
           Barry R. Gorsun. (Originally filed as Exhibit 10.17)(1)

10.17  --  +Promissory Note, dated February 12, 1993, by Barry R. Gorsun. 
           (Originally filed as Exhibit 10.18)(1)

10.18  --  Promissory Note, dated April 1, 1991, by Summa Four, Ltd. 
           (Originally filed as Exhibit 10.19)(1)

10.19  --  +Promissory Note and amendment thereto, dated April 11, 1989, by
           John T. Boatwright. (Originally filed as Exhibit 10.20)(1)

10.20  --  Letter Agreement, dated October 15, 1992, by and between the
           Registrant and Fleet Bank of Massachusetts, N.A., as modified.
           (Originally filed as Exhibit 10.21)(1)

10.21  --  Customer Purchase Agreement, dated November 20, 1992, by and between
           the Registrant and Sprint/United Management Company. (Originally 
           filed as Exhibit 10.22)(1)

10.22  --  Specialty Switches Contract, dated March 13, 1992, by and between
           the Registrant and AT&T, Inc. (Originally filed as Exhibit 10.23)(1)
    
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                                      -2-

   
     10.23  -   Customer Purchase Agreement, dated September 11, 1991, by and
                between the Registrant and United States Advanced Networks.
                (Originally filed as Exhibit 10.24)(1)
     
     10.24  -   Customer Purchase Agreement, dated October 10, 1990, by and
                between the Registrant and Unisys Corporation. (Originally filed
                as Exhibit 10.25)(1)
     
     10.25  -   Resale Agreement, dated November 5, 1992, by and between the
                Registrant and IBM Canada Ltd. (Originally filed as 
                Exhibit 10.26)(1)
     
     10.26  -   Agreement, dated October 14, 1992, by and between the Registrant
                and Claircom Communications Group, L.P. (Originally filed as 
                Exhibit 10.27)(1)
     
     10.27  -   Release, Settlement and License Agreement, dated November 2,
                1992, by and among the Registrant, Omnital Corporation, Aspect
                Telecommunications Corporation and Richard L. Scully. 
                (Originally filed as Exhibit 10.28)(1)
     
     10.28  -   License Agreement, dated December 1, 1983, by and between the
                Registrant and Industrial Programming, Inc. (Originally filed as
                Exhibit 10.29)(1)
     
     10.29  -   Product Development and Licensing Agreement, dated January 19,
                1990, by and between the Registrant and The Telephone
                Connection, Inc. (Originally filed as Exhibit 10.30)(1)
     
     10.30  -   +Letter Agreement, dated June 16, 1993, by and between the
                Registrant and Mr. Mel R. Ethan. (Originally filed as 
                Exhibit 10.31)(1)

     10.31  -   +Severance and Settlement Agreement and Release, dated August 8,
                1994, by and between the Registrant and James J. Fiedler.
                (Originally filed as Exhibit 10.33)(4)

     10.32  -   +Agreement, dated August 1, 1994, by and between the Registrant
                and Barry R. Gorsun. (Originally filed as Exhibit 10.34)(4)

     10.33  -   Loan Modification Agreement, dated July 28, 1993, by and between
                the Registrant and Fleet Bank of Massachusetts, N.A. (Originally
                filed as Exhibit 10.32)(2)

     10.34  -   Promissory Note, dated July 28, 1993, by and between
                the Registrant and Fleet Bank of Massachusetts, N.A. (Originally
                filed as Exhibit 10.33)(2)

     10.35  -   +Amendment dated August 31, 1994, to Lease Agreement dated
                July 18, 1990 with Northern Manchester Trust. (3)


     10.36  -   Amendment, dated August 2, 1994, to the Letter Agreement dated
                October 15, 1992 by and between the Registrant and Fleet Bank of
                Massachusetts, N.A.

     11     -   Statement Regarding Computation of Per Share Earnings. (3)
     
     21     -   Subsidiaries of the Registrant. (3)
     
     23     -   Consent of Coopers & Lybrand. (3)
    

------------
   
(1)  Incorporation herein by reference to the Registrant's Registration
     Statement on Form S-1 (File No. 33-66602) as declared effective by the
     Commission on September 23, 1993.

(2)  Incorporation herein by reference to the Registrant's Amendment No. 2 to
     its Annual Report on Form 10-K for the fiscal year ended March 31, 1994,
     filed with the Commission on June 19, 1998.

(3)  Incorporation herein by reference to the Registrant's Annual Report on 
     Form 10-K for the fiscal year ended March 31, 1995, filed with the
     Commission on June 27, 1995.

(4)  Incorporation herein by reference to the Registrant's Amendment No. 1 to
     its Annual Report on Form 10-K for the fiscal year ended March 31, 1995,
     filed with the Commission on May 9, 1997.


+    Management contract or compensatory plan or arrangement filed as an
     exhibit pursuant to Item 14(c) of this report.

(5)  Incorporation herein by reference to the Registrant's report on Form 8-K
     filed with the Commission March 6, 1995.
    

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                                  SIGNATURES

     Pursuant to the requirements of the Securities and Exchange Act of 1934,
the Registrant has duly caused this amendment to the report to be signed on its
behalf by the undersigned thereunto duly authorized.


                                        SUMMA FOUR, INC.

 

                                        By: /s/ Jeffrey A. Weber
                                            -----------------------------
                                            Jeffrey A. Weber
                                            Vice President
                                            and Chief Financial Officer 

Dated: June 19, 1998