Navigate to the settings section in the Management Interface and select Field Definitions under Custom Fields. Create a New Field, perhaps called PhoneNumber, and check 'Applies to only end-users and employees.' Then under Tickets select Signatures and edit the template you're wanting to add signatures to. Set your cursor where you want the phone number to appear and drop down the list of variables in the editor. You should see your newly created variable among the list, select it. Now the only thing left to do is go to the Employees section under Organization and edit the employees you want to add phone numbers to. You'll notice the Custom Fields tab now has the field you created in the first step for phone numbers. Once filled out it will insert that value into the location you inserted the variable into the template.