Hi, @harryparker12 .
This is Max from the Shopify Team.
Much like the customer email, the customer’s phone number also serves as a unique identifier and, currently, two customers cannot have the same telephone number.
When entering the number in this field ensure it’s being entered in the international format and the length of the number is correct for the international prefix in use. For example, an Australian number would require the international prefix +61.
Have you tried removing the phone number from the old Customer account to then allow you to create a new edited profile for the customer? Check out this page for some details on how to make edits!
Let us know if that helps you make changes that best suit your customer, or if you have any more questions.
Cheers,
Max