I have nothing but good things to say about Truphone, but its technology is actually easier to block. TMobile's connection looks like a VPN connection. Most public hotspots cannot afford to block that. SIP is often blocked.
Certainly there are countries that block VOIP and it would be easy for them to target TMobile as well. Last I checked, however, this wasn't the case. Trying to use VOIP in a banned country is a cat and mouse game and you can't count on the cat staying put.