Specified telephone number not allowed

What causes this issue?

The telephone number must be selected from the list of deflect numbers.
Other telephone numbers cannot be entered here.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Select another telephone number or inform your system administrator
that the number you require should be inserted in the list of valid
numbers.

Error message extract from SAP system. Copyright SAP SE.