error invalid number

We understand the importance of being able to send messages, @DavidSW420. Let's work together to determine why you're receiving this error. 

It's fine for you to use an international SIM in your device, but it's no guarantee that it won't cause complications. It could have messed with your network settings and retained the 11 digits in memory. Have you tried doing a network reset? Go to Settings > General > Reset > Reset Network Settings. 

For specific instructions on how to reset your device, visit our Device How-To. Select your device's brand or model > Your Device > Tutorials > Troubleshooting > Reset Device.

Once your network settings have been reset, try deleting the contact and adding it back. Another thing we'd recommend is erasing the thread the error occurs on and starting a new thread. 

Please let us know how this works. We're happy to provide additional assistance if needed.

Sydne, AT&T Community Specialist

This problem has been around for at least 7 years and there seems to be no solution. At least on all the help sites and forums.

It is criminal that this is still a problem.

Yeah, there is.  Reset network settings as instructed 

Thanks, but doesn't work.