The purpose of this document is to show how SIP works to setup a call from one device
to another. Below is an example of what SIP packets are exchanged between two devices
and a server.

Example 1.
Directory number 50001 calls 50002

1) Phone device with directory number 50001 calls device with 50002 via the SIP server, 50001 sends an INVITE packet.

1b) The server lets caller 50001 know the server sent the request on with a '100 Trying' packet and passes the INVITE method packet to the other device.

2) 50002 lets the caller know the phone is available and recieved the INVITE request with a '180 Ringing' packet.

3) the called picks up the line for converstation, so a '200 OK' is sent back to let the caller know the session has been answered.

Example 2.
Directory number 50001 calls 50002