The zaurus may be good for this - I don't know since I've never owned one (despite wanting to - they're like hen's teeth around here). But if you're after new kit for such a project the flavour of the month must be Raspberry Pi. How it interacts with your phone service will depend largely on where you reside so you could do worse than investigate some local VoIP resources for info on that.

Good luck with it and be sure to report back when you have it up and running.