Deregister iMessage Form Does Not Accept Valid Phone Number

Using my valid ten-digit phone number with a 279 area code, the "Deregister iMessage" form does not work and states "Please enter a valid phone number." The area code 279 became active in 2018 so I assume the form must be validating against an out-of-date list of area codes. The form needs to be updated with a current list of area codes, or simply not try to "validate" against a list of area codes which is constantly changing.

In the meantime, contacting Apple Support by phone has been no help in deregistering iMessage, and directed to contact the carrier instead. How can I deregister iMessage when the form will not accept my valid phone number?