Another option is FreeSWITCH, an open-source telephony platform with development led by a Barracude Networks guy. I only learned about it yesterday, so I can't make a recommendation. http://www.freeswitch.org/
I used FreeSWITCH extensively for the telephony for some spoken language processing research. It is easily customizable, but has sensible defaults and is quite full-featured. Awesome software - I highly recommend it.