Sending SMS: Phone Country Required Error

See issue below. One customer reporting they are getting this error when trying to send SMS. Didn't happen every time. They said:

"Today, when attempting to send SMS messages a few times, we encountered an issue. I'm attaching a screenshot below. This message popped up several times, preventing us from sending the SMS. For some numbers, the issue was resolved when we refreshed the page and attempted to send the message a second time, but for others, it was not resolved. This issue did not affect all the numbers but appeared quite frequently today, and it hasn't happened before."