SIP Express Router (ser) is a high-performance, configurable, free SIP (RFC3261) server.

http://www.iptel.org/ser/

Start:
/opt/sbin/ser -P /var/run/ser.pid -m 5

Stop:
kill `cat /var/run/ser.pid`

Start in debug mode:
/opt/sbin/ser -P /var/run/ser.pid -m 5 -DEddddddd