We ran openSIPS before but we found the pace of development to be higher on Kamailio in general. Plus most of our customers are running it instead of openSIPS.

We ran openSIPS before but we found the pace of development to be higher on Kamailio in general. Plus most of our customers are running it instead of openSIPS. We're currently trying to build the world's smallest cluster; seven raspberry pi's will hopefully support 100 phone calls or so with no single point of failure. If you're looking for something single server you want freeswitch by itself. That thing is a beast if you just want call processing and don't need carrier grade reliability. We made a frontend called BlueBox which is a very popular distribution; we're revamping it for Cluecon (hopefully we'll have it out by then) so if you're going to try this I'd recommend waiting until the new version comes out. Lastly, there's a lot of research going on at FreeSWITCH about running it by itself on the Pi. I believe it is workable now, you'll probably find a bunch of info with a quick google search.