Hello.
In most cases you can work round this problem with a restart of the phone. In some cases, that's not enough, and a reset of the network settings is required instead.
It's best to try the restart first as it's quick and easy. The network settings reset is most likely to fix the issue, as it will cause the iPhone to need to redownload the carrier settings, but it also means you might have to set up your WiFi networks on your iPhone again.
Pete.
