App says "Please enter a valid phone number"

While signing up the minitoring (Setting --\> Monitoring), Ring app keeps asking to enter valid phone number eventhough it is a right number. (in xxx-xxx-xxxx format). Any idea why this happens? The field only allows number (only number keypad shows up in the app and +1 is already pre-filled), so I don't know why it complains. Any idea on how to bypass this?