Nokia :: Will An Unlocked ATT Phone With Another Provider

Knowing the right way to pose this question is probably harder than answering it.I want to use an ATT Nokia 6750 phone in Mexico with cell provider Telcel. I have to unlock the phone. No problem. Lots of places in town will do that for a fee. I also want to buy Telcel's 3G service and use it on the same phone. A possibly uninformed Telcel agent says it's impossible because the phone has ATT's software not Telcel's. I suspect he is wrong.

Will an unlocked ATT phone with a Telcel SIM successfully connect to a 3G network OR is there some Telcel software gatekeeper to prevent this? Do you suppose he is just talking about the Access Point configuration? Any ideas? Partial answers are often helpful it you don't have the full answer.