Sometimes when I replied to text messages from certain people in my address book, I would get the following error “Error invalid number. Please re-sending using a valid 10 digit mobile number”. Even when the contact number WAS a valid 10-digit number (instead of assuming the local area code).
After a couple months of bounced messages, I finally found the fix: just send yourself a text message. No need to delete the contact and recreate, or only reply to text messages (source).
Charlie Cleveland 