People have reported that it is good enough to work with the G729 low bandwith codec. I'm not sure whether Vonage uses it but many other VOIP providers do. Because you have to pay some licensing fee per client for this codec, many carriers avoid it. VOIP.MS is one cent a minute and supports it. You can purchase the codec with CSimple (Android) and Groundwire (iOS). I am not sure how TMobile plays with SILK. I am sure that GSM will work but it is not the best sounding codec.