Step 1: If you haven’t already, insert your AT&T SIM into your iPhone 4S.
Step 2: Make a drop-call to 611, which is AT&T’s customer service number. Once you’ve dropped the call, activate Airplane mode.
Step 3: Remove your AT&T SIM and replace with the T-Mobile one
Step 4: Ensure that Wi-Fi is deactivated completely by ‘forgetting’ any automatically connected...
