Can't speak specifically to your question, other than say we are on Rogers and we used to be able to send and receive texts without any change to the phones (worked w rogers sim) up until a couple years ago. Now despite having a cell signal, Rogers doesn't work (for us in Santiago area) and we understand it had something to do with them not signing a agreement w EtCetsa (sp?).
One of our Rogers phones is locked, the other, unlocked, so that wasn't the issue either.
Telus on the other hand, does work. We just add a international text pkg for 20.00, good for the month, allowing unlimited incoming texts, and around 50-60 outgoing.
We did learn when we went over the outgoing limit it was because sending a text more than 140 characters will count as 2 (or more) texts toward your 50-60 limit.
Update: I see now Telus offers a 'travel pass' for 15.00 for unlimited incoming, and 75 outgoing.
Might be worth seeing if MBT offers something similar, to avoid any hassles with getting a cuba sim/pkg for such a low cost.