I use VoIP all the time, Callcentric. I've never had problems using my iPhone and Acrobits (using G.711u and G.711a audio codecs) and making calls to US/Canada

I use VoIP all the time, Callcentric. I've never had problems using my iPhone and Acrobits (using G.711u and G.711a audio codecs) and making calls to US/Canada numbers. As long as my WiFi connection is good then the call is great as they use the same Tier 1 carriers everyone else uses in the US. If you're having problems with call quality it's probably an issue with your connection.