EXHIBIT 2.03 ADDITION OF PARTY TO AGREEMENT AND PLAN OF MERGER WHEREAS, pursuant to the Agreement and Plan of Merger (the "Merger Agreement") dated as of February 25, 2000 between Mustang.com, Inc., a California corporation (the "Company"), and Quintus Corporation, a Delaware corporation ("Parent"), as amended by the Company and Parent on May 8, 2000, Mustang.com Acquisition Corporation ("Merger Subsidiary"), a wholly-owned subsidiary of Parent, was duly incorporated and organized in the State of Delaware on May 9, 2000, and Parent, the Company and Merger Subsidiary agree that Merger Subsidiary should become a party to the Merger Agreement as a constituent corporation. NOW, THEREFORE, in consideration of the agreements contained herein and other good and valuable consideration, the parties hereto agree as follows: Merger Subsidiary hereby becomes a party to the Merger Agreement as a constituent corporation and agrees to be bound by the terms of the Merger Agreement, as amended, applicable to Merger Subsidiary. IN WITNESS WHEREOF, the parties hereto have caused this agreement to be duly executed by their respective authorized officers as of the 9th day of May, 2000. QUINTUS CORPORATION By: /s/ Alan K. Anderson -------------------------------------------- Name: Alan K. Anderson Title: Chairman and Chief Executive Officer MUSTANG.COM, INC. By: /s/ James A. Harrer ------------------------------------------- Name: James A. Harrer Title: President and Chief Executive Officer MUSTANG.COM ACQUISITION CORPORATION By: /s/ Alan K. Anderson ------------------------------------------- Name: Alan K. Anderson Title: President