In Settings> Work Orders> Custom Cells you can set a cell to a Contact's Phone number for a Customer Contact or a Job Site Contact. (further down than shown in the choices is the Job Site Contact Phone number)
if you create a column like this then in the work order you will be able to choose from phone numbers for customers and linked contacts:
Once selected, the entry shows a link that will popup full contact information for the customer or contact with clickable links to make phone calls:
Clicking any of the phone numbers in this dialog will dial the number.