Using letters in a phone number as a mnemonic, or just to flow better, is not a "rare case." You should revise your regex to accept letters, then whether you store them internally as letters or convert to the corresponding numbers is up to you.
That makes sense if you are making a contact book or social profile. I was focusing on the case where I want the number as contact information. I want to validate up front that it is a number that I can actually call and don't really care about how the user wants to format it.
I am letting you. It is just when the number is sent to the backend it is converted into a usable format where I can for example generate tel: links and otherwise use the number. I agree that there is some value to being able to each your number back how you typed it, but I'm not sure it is more valuable then showing you the number how I am actually going to try to call it.
Sorry, sounds like a misunderstanding. I always meant to accept anything, that is why I put a replace in my top comment not saying that I would reject the form.
