My phone is not new, but I have not roamed before, unless I've done that in the US and not realized it. Should I basically talk to to TMobile (my US carrier) and see what they have to say?

You have to specifically request international roaming/dialing be added to your account. It doesn't cost anything extra to add, but they do run a credit check on you again. If all is well, it only takes the CSR about 1 minute to put that on your account. Dial 611 and tell them that's what you want.