Aria have a great deal on at the moment for USB VOIP handsets for £12.87, see the Deal Of The Day page model TEL-E-ZI-PHONE-S, you can get OSX drivers from here.

It seems to work really well, maybe a little buggy at the moment but they are in Beta. I don't think I have seen they cheaper anywhere else?

I am using on of their PSTN split boxes at my end so I can use the same handset as PSTN, there are no OSX drivers for this yet the quality seems alittle poor, a little too much sidetone for me, but hey I guess I have an ear for that sort of thing

Cheers

Cris.