There is no guarantee that these troubleshooting options will help, but they did helped most users.
Turn off Wi-Fi Calling
Go to the Settings app on your iPhone
Toggle Wi-Fi Calling on This iPhone to off
Afterwards, try the methods again
Reset your iPhone Network Settings
Go to the Settings app on your iPhone
Scroll down to Transfer or Reset iPhone
Tap Reset Network Settings
Afterwards, try the methods again
WARNING: This troubleshoot option is for the last resort! This will ease the ENTIRE iPhone! Please backup your device before proceeding!
Factory Reset the iPhone
Go to the Settings app on your iPhone
Scroll down to Transfer or Reset iPhone
Tap Erase All Content and Settings
Afterwards, set up your iPhone and try the methods again