My iPhone Is Unlocked But Won't Read My SIM Cardby Robert SchraderUpdated September 28, 2017
Troubleshoot an Apple iPhone that is unlocked but won't read a SIM card that doesn't come from the carrier who originally sold the phone. If you properly unlocked an iPhone, it accommodates any SIM card with an active service plan. The case of an iPhone not reading a SIM card means that it was either improperly unlocked or that the SIM is faulty.
The Unlock Procedure
In spite of the fact that both AT&T and Apple strongly discourage unlocking or "jailbreaking" iPhones, the underlying procedure is nonetheless simple. Specifically, you download a software application onto your computer, transfer it onto your iPhone and run the app from the phone. After the iPhone restarts, it is unlocked and ready to be used with any SIM card. Another sign an iPhone has been unlocked is the presence of Cydia, a third-party application store that sells apps and other modifications not available in the official AppStore.
After you unlock an iPhone, nothing conspicuous about the device changes, except for the appearance of Cydia. Although unlocking an iPhone allows you to use the device with any SIM card, it doesn't prevent you from using the phone with AT&T. If you want to begin using the device with a different carrier, you must turn off the phone, replace the SIM card -- doing so involves using the sharp end of a paperclip to pull out the SIM "tray," then replacing your current SIM with a different one -- and turn the phone back on. If the carrier's name appears in the upper, left corner, unlocking was successful.
If the phone doesn't register the non-AT&T carrier after you insert the SIM card and restart the phone, several factors may be to blame. For example, if you haven't yet activated service on the SIM card you insert, you aren't able to make calls, send texts or use the Internet via the new SIM, even if the carrier's name appears. If inserting a non-AT&T SIM produces a "No Service" error, the unlock procedure failed and you must redo it. Another way to test whether the phone is successfully unlocked is to insert a friend's SIM card into it. If the phone works, then the unlock was successful.
One impediment to maintaining an unlocked iPhone is the fact that updating either the phone's operating system or carrier software re locks the phone to its original carrier. Whenever a software update is available, iTunes prompts you with a "Yes/No" option. If you choose "Yes" and install the update, the phone automatically re-locks, without notifying you. If you recently updated software and aren't able to use the phone with a non-AT&T SIM, this is the reason.