It is validated in the 7 places:
Where are the telephone validation rules declared?
How to change customer address frontend validation rules?
Which xml config is responsible for the validation rules etc. for "billing data" form in one step checkout (2-nd step)?
Why does a saving a customer's address in his address book not work in my store?
This is brilliant information! The telephone field should be made to be optional or compulsory in the backend.
How would I use the information here to make the field optional and update safe?
Hey! you forget: