Did you have a roaming pack/roaming enabled?
I’ve had that happen a few times - iMessage goes from email rather than phone number. Last time (a few months ago) and Optus didn’t roam at all in Uruguay. Even though I had Flexiroam for data, I could get iMessage back until we got to Argentina. Took some fiddling in the phone settings (make sure aíralo is set as the default line for calls) and going into iMessage settings and ensuring your phone number is ticked to initiate iMessages.
