[AC Question]

I was able to call a few days ago, but then every time I call, I get that message. I can use my data and I receive text messages (I can't send any though, it says "message failed to send") just fine.

[Javier P]

Have you tried rebooting your phone? You can can also remove your SIM card while your phone is off and insert back again.

[Rukbat]

If all else fails, bring the phone to the carrier. SIM cards don't often fail (I have some 11 year old ones still working), but they can fail. The carrier should replace bad ones for free.