I moved from Vonage to Viatalk almost 2 years ago. They let you know the SIP-credentials and they do work on N900. They do transfer the number over from your Vonage provider. (I did that...) So far I have been happy with their service and have not had any outages that I had noticed.

The only "problem" is that I can not have both my home SIP phone adapter and N900 connected simultaneously. So the simultaneous ring of different SIP devices using same credentials does not work. (This does work with Gizmo5.) However, Viatalk do have simulring options that you can setup up any other ordinary phone.

Voicemail of course is included. Also E911 service if that is of interest.

For pricing it (with unlimited calling) is less than $10 a month at the moment ($99/year).