<PAGE>   1
                                                                   EXHIBIT 10.18

                         OBJECTIVE COMMUNICATIONS, INC.
                             50 INTERNATIONAL DRIVE
                         PORTSMOUTH, NEW HAMPSHIRE 03801


                                February 2, 1999


              RE: AGREEMENT REGARDING $3.125 MILLION AGGREGATE PRINCIPAL AMOUNT
                  OF 5% CUMULATIVE CONVERTIBLE DEBENTURES DUE 2003

Dear    : 


       This letter agreement is to set forth our mutual agreement and
understanding with respect to the outstanding $3.125 million aggregate principal
amount of 5% Cumulative Convertible Debentures due 2003 (the "Convertible
Debentures") of Objective Communications, Inc., a Delaware corporation
("Objective" or the "Company"). The undersigned investors (collectively, the
"Investors" and individually, an "Investor") currently hold $2.5 million
aggregate principal amount of the Convertible Debentures. Each individual
signing this letter agreement on behalf of an entity that holds Convertible
Debentures represents and warrants to Objective that it is authorized and has
full authority to execute, deliver and enter into this agreement on behalf of,
and to bind, such investor. This agreement shall supercede our letter agreement
dated January 13,1999.

       As you are aware, Objective is negotiating an immediate bridge financing
and a subsequent equity financing, and the investors in that financing have
requested the Investors to agree to certain changes in their rights with respect
to the Convertible Debentures.

       In consideration of the mutual covenants and obligations set forth in
this letter agreement, and in the other documents and agreements among
Objective, the Investors, and the other management investors who purchased
Convertible Debentures, Objective and each of the Investors, agree as follows:

       1.     Each Investor agrees that it will not exercise its right to 
              convert the Convertible Debentures to common stock, par value 
              $.01 per share of Objective (the "Common Stock"), until the date 
              on which a public or private equity financing 


<PAGE>   2


                                                                        - 2 -

              with gross proceeds to Objective of not less than $8 million (a
              "Qualified Financing") is consummated.

       2.     Upon the closing of a Qualified Financing, the principal amount of
              and accrued and unpaid interest on the Convertible Debentures will
              automatically and without any further action on the part of the
              holder convert into the type(s) of securities issued in the
              Qualified Financing at the conversion price set forth below. By
              way of example, if Objective issues and sells shares of Common
              Stock in the Qualified Financing, then the Convertible Debentures
              will be converted into Common Stock, and if Objective issues and
              sells units consisting of Common Stock and warrants to purchase
              shares of Common Stock in the Qualified Financing, then the
              Convertible Debentures will be converted into such units.

       3.     If the Qualified Financing is a public offering ("Qualified Public
              Financing"), the number of securities issuable upon such
              conversion will be equal to the principal amount of the
              Convertible Debentures, plus accrued and unpaid interest thereon,
              divided by a "conversion price" where the conversion price will be
              equal to the lesser of (i) $2.50 per share, or (ii) 75% of the
              price at which the securities were sold in the Qualified
              Financing.

       4.     If the Qualified Financing is a private financing ("Qualified
              Private Financing"), the number of securities issuable upon such
              conversion will be equal to the principal amount of the
              Convertible Debentures, plus accrued and unpaid interest thereon,
              divided by a "conversion price" where the conversion price will be
              equal to the price at which securities were sold in the Qualified
              Financing. In addition, upon the closing of a Qualified Private
              Financing, Objective will issue to the former holders of the
              Convertible Debentures the number of warrants to purchase shares
              of Common Stock equal to 20% of the shares of Common Stock
              issuable upon conversion of the Convertible Debentures. Such
              warrants shall have a five-year exercise period and an exercise
              price per share equal to the market price per share of Common
              Stock on the date on which the warrants are issued.

       5.     In the event of a Qualified Public Financing, the shares of Common
              Stock issuable upon conversion of the Convertible Debentures will
              be registered at the time of such offering. In the event of a
              Qualified Private Financing, the shares of Common Stock issuable
              upon conversion of the Convertible Debentures will be registered
              promptly as practicable following the conclusion of such
              financing, but in no event will such shares be registered later
              than the time at which the shares issued in the Qualified Private
              Financing are registered or the end of the six-month lock-up
              period. The holders of the Convertible Debentures also agree to
              hold the shares of Common Stock issued upon conversion of the
              Convertible Debentures, in the case of a Qualified Public
              Financing, for a period of at least twelve (12) months following
              the effective date of the Company's registration statement that
              relates to the Qualified Public Financing and, in the case of a
              Qualified Private Financing, for six months following the date on
              which the Convertible Debentures are converted.


<PAGE>   3


                                                                        - 3 -

       6.     The holders of the Convertible Debentures also hereby waive any
              default by Objective with respect to the failure to register the
              shares of Common Stock issuable upon conversion of the Convertible
              Debentures for resale by the holders thereof on a registration
              statement filed with the Securities and Exchange Commission (the
              "Commission") and to maintain the effectiveness of the
              registration statement. The undersigned holders of the Convertible
              Debentures hereby waive any right to have the Convertible
              Debentures registered on a registration statement filed with the
              Commission prior to the Qualified Financing. In exchange for such
              agreement, the Company agrees that if the Qualified Financing is a
              public offering, then it will include the shares of Common Stock
              issuable upon conversion of the Convertible Debentures in the
              registration statement filed with the Commission with respect to
              the Qualified Financing, and to use its best efforts to have the
              registration statement effective at the time at which the lock-up
              agreement described above expires.

       7.     The undersigned hereby consents to the amendment of the
              certificate evidencing the Convertible Debentures and/or the
              Subscription Agreements and/or other documents relating to the
              issuance of the Convertible Debentures to the extent necessary to
              incorporate the above agreements, and, promptly upon the request
              of the Company, agrees to execute and deliver such amendments to
              the Company.

       8.     In the event that the Qualified Financing is not completed on or
              before June 30, 1999, then this letter agreement shall be null and
              void and of no force and effect.

       This Agreement shall be effective upon and as of the date and time at
which the Company consummates a bridge financing and receives gross proceeds
from a bridge financing in an amount of not less than $1 million. All numbers
and share amounts set forth in this letter agreement are based on the current
number of shares of common stock outstanding, and shall be adjusted for any
future stock splits, stock recapitalizations, reverse stock splits or similar
changes in the Company's capitalization.

       If you are in agreement with the foregoing, please sign and return one
original copy of this letter to the undersigned at Objective Communications,
Inc., 50 International Drive, Portsmouth, New Hampshire 03801 by overnight mail
and by telecopy (Telecopy number: (603) 334-2212). If you have any questions,
you can reach me at (603) 334-6741.


                                   Sincerely,



                                   Robert H. Emery,
                                   Vice President, Finance and Administration
<PAGE>   4

                                                                        - 4 -



       The undersigned authorized representatives of the Investors listed below
hereby acknowledge and agree to the terms and conditions of this letter
agreement and agree to be legally bound by its terms.


Dated: February 2, 1999                  
                                         By:    
                                         Its:   


                                         By:    
                                                ------------------------------
                                         Name:  
                                         Title: