A friend gave me an iPhone 4 after he bought an iPhone 5 and everything works except the parts that need a SIM card. I put a SIM card in from my old phone but there is no signal, in the top left corner of the iPhone 4 it says “No Service”. Do I need to register the SIM card to the iPhone somehow? And if so do I need to get a new SIM card from a network that Apple has approved? I live in the UK, do I need to be on O2?
To use the phone with another SIM, the device has to be unlocked by the provider to which it has been locked to before you got it. Contact them and give them the details like IMEI, serial number etc. If unlocking is supported by the carrier, they will pass this info on to Apple, and after a certain time, you can restore the phone and use it with a different SIM.