I'm using SJ Phone with my credentials from ViaTalk on my XV6700 and it works pretty well. There is a beta that mostly supports WM5 with a couple of bugs, such as sound comes from the speaker vs the earpiece (but I use BlueTooth, which works fine), and several of the key fields (3 IIRC) that you need for your SIP server info don't have labels, so you need to look on-line to know what goes in each one. Once set up, it works great with a WiFi connection - very good voice quality. I've tried with EVDO connection, but it has very poor voice quality that way. The version I'm using is Build 1.62.320b, Feb 15 2006.