Contact Info: Phone Area Code validaton not working as expected

Summary:

I have a requirement to add additional validation to the Me > Personal Information > Contact Info screen in Oracle Fusion HCM as the Area Code input field is not applicable for UK Mobile Numbers. We already have had some staff populate values in the area code so my requirement is to show an error message asking the user to remove the area code should they edit an existing number, however this is not working as I expect.

Content (please ensure you mask any confidential information):

Above: User edits an existing mobile number which previously had an area code populated. My custom error message displays correctly prompting the user to remove the area code.