No voice or msg roaming. What gives?

[Hawaii Built]

My Sensation used to jump on ATT's network when I am outside TMo's signal automatically - (I have the data turned off when roaming). Recently, when I have no signal and try to manually switch to ATT, I get an error message to the flavor of my SIM not being compatible to that network.

Did I inadvertently change one of the phone's settings? Any way to get voice back outside of TMo's network?