Add phone number inside Ticket form

Hello,
I have to show the customer’s main phone number (not the contact one) inside the Ticket form, but I can’t find how to add a field from another module.

Can anybody help? Thanks

Can you provide some more detail of your full requirement, ideally with some screen shots?

Thanks,

Will.

Maybe I used wrong terms: I have to show the Account’s main phone number inside the Case form, e.g. where I put the arrow in the attached image.

Thanks again