Many of the developers who were focused on User Agents continued with those parts of the original code base as the sipXtapi project.
As noted above, this project began with contributions from Pingtel. Pingtel began life focused on the development of VoIP phones, and quickly narrowed that focus to SIP phones. The Pingtel Expressa phone was considered one of the best SIP phones in the business at one time, and nearly every SIP interoperability lab had at least one for testing (some probably still do). It was, alas, not a commercial success, and Pingtel eventually shifted its focus to the SIPxchange PBX product and the accompanying open source projects as described above.
In July of 2007, Pingtel was acquired by Bluesocket, which in turn sold all the assets of Pingtel to Nortel in August of 2008. Nortel had already been selling a product (Software Communications System - SCS) based on code from the sipXecs project for some time. When Nortel bought the assets of Pingtel, they also hired all of the developers from Bluesocket.