Display Alt. Phone if the Main Phone is Blank

1. Navigate to Customization > Lists, Records, & Fields > Entity Fields > New.
2. Enter Label.
3. Uncheck the Store Value field.
4. Under Applies To tab select Customer.
5. Navigate to Display tab and select Main under Subtab field.
6. Under Validation & Defaulting tab set the Default Value = COALESCE({phone},{altphone}).
7. Set Formula = T.
8. Click Save.

Note: This can be done the other way around as well. I.E. displaying the main phone number if the alt. phone number field is blank.