I really couldn't understand what you meant, but I'm guessing you're buying a phone with an x provider subscription, and want to use a y subscription with it, no?
If that's the case, most of the time the phone is locked against the other providers, meaning it doesn't get any reception with the y provider. You'd need to get it cracked/hacked/something, which voids the guarantee.
But that's the case here in Turkey (and England, I've learned), maybe it's different over there.
On the other hand, if this wasn't what you were asking..well..then I feel like an idiot
