>>You can split it into two textboxes and label. Anyway it's better to store >>extension into separate field because not all phones would have extensions.

Obviously, I can separate the field into two textboxes but that was not the question. Changing the back end database is not an option.

>>Anyway it's better to store >>extension into separate field because not >>all phones would have extensions.
It is not a better data design to store the extension as a separate field. There is no use for the phone number without the extension and there certainly is no use for an extension without a phone number.