May you please provided more information, that way we can see what can be done.
What phone is it:
Software version: (please check for update)
Is it everywhere you go?
Restart your phone,
Check account status.
More steps that you can try:
Remove phone from case,
Make sure you SIM card is not damaged,
Disable WiFi Calling,
The more info you give the faster we can figure out a way to fix it.