I'm running Incredible PBX 3.6 on a 256MB while I await the arrival of the 512MB version.
I have to say, it's quite brilliant. I'd been struggling to find a real practical use for my Pi's; now I have a comprehensively featured PBX, integrated with my analogue as well as VOIP services. Thanks to some careful scripting, the numerous pesky sales/scam/survey calls I used to get every day have vanished.
Took me three days to get my head around it, but I'm a seriously happy bunny, now