On Cisco Packet Tracer, how do I setup a VoIP network with 2 routers?

I am trying to setup a VoIP network on Cisco Packet Tracer. I know how to configure telephone service on the 2811 router. But I would like to have 2 routers with telephones connected to both routers, and I would like each phone on each router to communicate with every phone. In other words, let's say that I have a router named Router-A and another router named Router-B. I want telephones connected to both routers, and I want a phone connected to Router-A to be able to call Router-B, etc. Please tell me in detail which commands I should use, and which routing protocols I should use.