Remove and then reinstall the SIM card on your smartphone to rule out the potential of a SIM card issue causing your iPhone to constantly ask you with this message. This will also offer your network services a fresh start and allow your SIM to connect to your wireless carrier's network towers for the first time. To correctly remove and reinstall a SIM card on your iPhone, follow these steps:

1. The SIM tray is beneath the Power button on the left side of your iPhone.
2. To eject a SIM card, insert a little paper clip or SIM ejector tool into the small hole at the bottom of the SIM tray.
3. Pull the SIM tray gently out of the way, then remove the SIM card from the tray.
4. Examine the SIM card carefully for any signs of possible damage, such as scratches or dents. Replace it in the SIM tray if there isn't one. Make that the SIM card is in the exact place it was when you withdrew it from the tray.
5. Replace the SIM tray after securing the SIM card in place.