My iPhone 5, three years old already, yesterday gave errors trying to place calls. After reboot, it won't unlock the SIM card, so it works, it can use Wifi and apps, but the telephone part is dead. I've rebooted several times with the same results. Do SIM cards go awry and produce errors like this? I can't try with another card since the iPad (same telephone number and connection) is larger than iPhone's. The cellular account should be good, the iPad can connect to the service provider.
Maybe a reminder to get a new one, however the 6 is too large, I love the smaller format of the 5