Cannot Call U.S. Phones from Mexico, even though I am paying for that

I had a post-pay Magenta plan and my phone worked just fine in Mexico, could call the U.S. no problem

Today I switched to Prepaid because its cheaper and they said I could still use my phone from Mexico, no problem, for an extra $5 /month

The problem is it doesn’t work. I cannot make phone calls to the U.S.

And I can’t even call Tmobile to fix this! The phone just says “this call cannot be made.”