Two templates are provided for PDF invoices.  Default templates is configurable via Invoice Settings and provide greater details.  A simplistic invoice template is also provided.  To use the simplistic template, click on Settings/Tools/Reports -> Invoice/General Settings.  Select "Use Simple Invoice Template" check box and click on the update button.  Under Edit Firm's Info option, enter the telephone number.  Telephone number will be displayed under your firm's information on the simplistic invoices.