I built my Call Forwarding app using Plivo. It was pretty straight forward to use. https://www.plivo.com/docs/getting-started/forward-an-incomi... is their official documentation on Call Forwarding. Also, every user gets a pre-built Call Forwarding which requires you just add the numbers for testing which looks like this - http://callforward.herokuapp.com/forward/?Numbers=NUMBER_1_H...