What would happen if I put a T-Mobile microSIM into a locked AT&T iPhone 4s?

I have an iPhone 4s locked on contract with AT&T. But I want to find out how T-Mobile's reception might be on HSPA+.So… If I buy a T-Mobile microSIM and put it into the iPhone, what would happen? Would it show me the bars and 4G for T-Mobile?

It won't work because since the phone is locked to AT&T, it will refuse to work with another sim besides an AT&T one. You'll need to get it unlocked to use both T-Mobile and AT&T's networks.

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