Validating Phonewords/Vanity Numbers

I am redesigning a system where the user can enter various contact details for associates via a personal details form.

In this form, there is a phone number input field. After a phone number is entered it appears in reading view as a clickable number. So it will dial the number for the user if clicked on.

There are a finite number of characters allowed in the field aside from numbers - hyphens, full stops, spaces, parenthesis, hash key e.t.c

Some people choose to input company contact numbers containing phonewords (or vanity numbers). For example - 18000 Call Sony

However some users are just flying through the form and putting letters in the phone number field that eg 1800 876 5432 (Call After 6) or 1800 876 5432 Ext450

I am trying to design a form that will accept phonewords as viable contacts (converting letters to numbers behind the scenes - and therefore making it diallable) but at the same time run a validation error against other letters/words that cause a system error when entered as part of a dial code.

The current issue is people are entering in a lot of unusable data in there

Phonewords (or vanity numbers)

e.g 1800 Call Sony

attached is an image of phone section of the form