Creating a Case for a Customer with No Phone Number

If a user will be creating a case (Lists > Support > Case > New) for a customer with no phone number and decided to add a phone number from the case record, the customer record phone field is updated by the number entered in the case by design.

This only applies to customer records with blank phone number fields. Customer records with phone numbers already will not be affected. So if a different number is entered in the case (the number on the customer record is displayed in the phone field on the case by default) when it is created, the number in the customer record is not changed.

Note: The same behavior is to be expected for the email field.