Hello Mike,
I can use FieldServices_JobSummaryV2 as an example. I'm going to make an assumption that you have been abel to get the report open in the Report Designer and go from there.
The good news it is simple to add a field to a report. With the report open, find the field you need in the Field List and drag it over to the Designer screen. Save and the field should now be visible. You can click 'Preview' to check out how the report looks.
The problem is Work Orders don't have the Customer's phone numbers in the table. They have only the contacts phone number. Work Orders are based on customers and Activities are for contacts. What you will have to do is when a Work Order is created, you have to go get the customer's phone number. This can be as easy as a new field on the activity table. You actually choose an 'Entity' when you choose a customer on a Work Order. (Entity is the parent table for customer and contacts table. For the work orders customer drop down, they are filtered by Entity type) Go to the Activity table and try adding the Phone field to the Activity. Then add the field to the Work Order screen (default is FieldService_AddEditWorkOrder).
Here is the final result. ( You can see the Entity Phone field. I have the two fields, Contact Phone Number and Contact Phone on the screen as well. They are used in Activities but not Work Orders).
I hope this helps,
Greg