It should not happen, this message, but it has been known to do this with people with unlimited calling plans @Jeffjiuxu . This should go away on it's own, and you can ignore it if the calls are completing.
But if calls are not able to be completed try to toggle into airplane mode then back to regular mode.
You may also try to reset your device's network settings. This also resets Wi-Fi networks and passwords, cellular settings, VPN and APN settings, and connected Bluetooth devices that you've used before.
To Reset Network Settings, begin by opening the Settings app. Next, tap General -> Transfer or Reset iPhone -> Reset -> Reset Network Settings. and enter your passcode.
Your iPhone will reset network settings and restart itself.
Find and tap Settings > System > Advanced > Reset options > Reset network settings.
If this and other suggestions are not working, submit a ticket to CSA for assistance.
To contact the Public Mobile Customer Support Agent (CSA)_Team, there are two methods to reach them found here: