Sorry, the validation for the phone field only allows characters that are used for phone numbers (numbers . ( ) – )
The only way to change that right now we be a hard code edit of the file where that form processing takes place. You would need to edit line 115 in the class-pta_sus_public.php file to allow any text.
If you are not comfortable doing that, please contact me privately via the contact form, and I’ll make a quick edit for you and send you the replacement file that will do what you want.