American Express has announced a strategic alliance with mobile pay firm Payfone. In addition, American Express will invest an undisclosed sum in Payfone. American Express will integrate its Serve digital payments platform into Payfone to let consumers pay for purchases from online merchants using on their mobile phone number. Consumers can choose to tether their mobile phone number to a variety of payment methods, including prepaid and postpaid mobile operator accounts and Serve accounts. For American Express, the Payfone integration is an important step in the international rollout of Serve.