As Cthulu suggested in the comments, I suggest trying out Asterisk. Only a few files (sip.conf and extensions.conf IIRC) need to be configured to have it up and listening for calls.
There's also an excellent free book that will help with terminology and configuration when just starting out. The book can be read in full online. You can also download a PDF version from O'Reilly.
There are also many flavours of Asterisk that have somewhat simplified interfaces to make setup easier for the first time. AsteriskNow can be downloaded directly from Digium (the company that's responsible for the Asterisk project). If you are going to be doing more SIP work in the future, it might be worth it to use 'real' Asterisk, but if this is a one time thing, try out AsteriskNow.
The other main project to investigate would be Freeswitch, though I find the Asterisk setup to be a little more straightforward.