said by Hagar: This is very interesting. I see that these services validate with Googlevoice. How do they get access to the GV credentials to validate you as a user?
It is the provider of the VoIP functionality that decides what is counted.
E.g. t-mobiles wifi app connects to T-mobile VoIP service which counts minutes.
Google voice app (not VoIP) actually use the cellular voice call to call/connect to google voice.
Talkatone, gtalk and grooveIP connect to google voice and t-mobile can not tell.
Vonage app connects to Vonage VoIP service which current give you 3000 free outgoing minutes t-mobile can not tell