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And command them to deactivate your old SIM. The third and final step? Activate a new SIM in your new phone.
Completing those three steps—wiping, deactivating your old SIM, and then activating a new one—ensures that your iMessages will get sent only to you and your iOS devices, and not anywhere else.
If talking to your transmitter is anathema to you, Hollington’s SIM PIN solution works, too. Read on.
Why a SIM PIN works, and why it’s perilous
Such a PIN SIM is separate from an iPhone passcode that you may have set. It specifically locks your SIM, and you’ll be prompted to reenter the PIN whenever you restart your iPhone. If you supervision padlock your SIM with a PIN and your phone ends up in the wrong hands, you’re more protected: After a remote wipe, the phone will restart and quick the new owner to enter the SIM before the phone will accept new iMessages (or FaceTime calls) at your old issue.
Besides the potential added annoyance of needing to enter your PIN code whenever you restart your iPhone, there’s one other potential downside to locking your SIM: It’s the tiniest bit dicey. There are a couple reasons for that: The iPhone’s interface for setting a new SIM PIN is rather awful, as we’ll rationalize in a minute. And you usually need to know your carrier’s default PIN before you can set a new one. Worst of all, if you prepare a mistake one too many times while changing the PIN or unlocking your SIM, you can end up locking yourself out—and at that point, only intervention from your shipper can help you.
Source: Macworld (blog)