Full OS X compatibility has been somewhat elusive when it comes to consumer VOIP handsets, but it seems to be arriving at last! If I had my own network connection (too much congestion on our house network) I would probably get one of these posh phones from IPEVO.
Apparently USRobotics 9600 phones can function as an audio device os OS X with these drivers. The IPEVO looks very nice though. Seems to be £25 here in the UK which I think isn't a bad price considering some actual design appears to have gone into it.