I have heard some conflicting reports about using the 3CXPhone for Android on T-Mobile in the US. I have a Samsung Vibrant (Galaxy S) and it works fine over WiFi, but on 3G it will register, on a call the remote side can hear me but I do not get any audio back. I have tried pretty much every combination of settings with no luck. It will work if I am remote and am using WiFi so it isn't a firewall issue. I then have used my laptop tethered to my phone and used the 3CXPhone for Windows client through my phone's 3G connection and I get the same problem. Enabling the tunnel fixes the issue. So it seems like this is a T-Mobile networking issue versus a hardware or firewall issue. If anyone has any success with T-Mobile US, I would love to hear about it.