T-Mobile prepaid use in Mexico observation.
 

In case you still have a 'Pay By The Day' plan with T-Mobile, be aware that they will charge the daily $1.00 charge if you use voice in Mexico. The rate is $1.49. It worked in Ensenada, BCN.

I don't know why you're surprised. The terms of PBTD are that you're charged any day you use voice minutes. You're using voice whether it's domestic or you're roaming. $1.49 is in addition to any other charges when you're roaming. If you're in Canada you'll pay 69¢/minute to roam in addition you'll pay $1.00.

I thought I had read somewhere that they did not charge the dollar if you were out of the US and thus paying the higher rates. But you are right of course.