Have you tried to remove the SIM card and try it in another phone? Did it work before? Is it intermittent or do you get this error all the time? Does self serve mention anything out of the ordinary? Just a few thoughts of what could be wrong...

Try resetting your phones’ network settings. This setting/feature is usually found under Backup and Reset. Or type ‘reset’ in the Settings menu search bar and look for reset network settings. 

Resetting your phones’ network settings does not erase any user data from your phone and restores cellular, Bluetooth and Wi-Fi settings to their factory default values. You will have to re-enter saved Wi-Fi passwords and Bluetooth devices will have to be re-paired (if applicable). VPN settings will also have to be reconfigured (if applicable). 

If the problem persists, it could be that your SIM card is showing signs of age and may need to be replaced. If you have access to a different phone, insert the SIM card into that phone to see if the SIM card works or is possibly at fault.