4701440 Error Invalid Number. Please re-send using a valid 10 digit mobile number or valid short code error message

Have seen online that this has been solved. Solution please? Have tried many options (delete account of person who's text always gives me error message, just enter the phone #) plus other options. Error message:

4701440 Error Invalid Number. Please re-send using a valid 10 digit mobile number or valid short code always pops up. Person can send me text. When I reply, same outcome