Found this interesting blog entry from Tony Rybczynsk of Nortel...
Asterisk may be older, but sipXecs is better
You have probably read about Nortel's Software Communications System 500 (SCS500), a Unified Communications (UC) SIP-centric software solution for SMB (30-500 users), and that our go to market includes IBM and Dell.
What you may not know is that the SCS500 is based on open source from SIPfoundry, and blends the best of both the open source framework and Nortel's experience and expertise in voice, data, multimedia and unified communications. Why did we choose sipXecs from SIPfoundry as the basis for SCS500?