Business Case:
When a Ship-to: Custom Address is chosen for a sales order in the Shipping and Billing tab, there is no ability to put a phone number. When it comes to courier shipments, they often times need a phone number for the ship to address in case there is a problem with the delivery.

Although the contact field is long enough to add a name and phone number it is not intuitive to users as normally contact means a person and phone no. means a phone number.