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The following messages are exchanged while UA1 uses a proxy server to connect to UA2:

1.

UA1 sends an INVITE request to UA2. This request is first sent to the proxy server and then to UA2.
2.

The proxy server responds with a 100–Trying message to UA1.
3.

The proxy server looks up user2's current location in a location service.
4.

The location service returns user2's current location.
5.

The proxy server decides to proxy the call and creates a new INVITE transaction based on the original INVITE message, but with the Request-URI in the start line changed to user2@alp.sev.com. The proxy server sends this request to UA2.
6.

UA2 responds with a 100–Trying message to the proxy server.
7.

The UA2 responds with a 180–Ringing response to the proxy server.
8.

The proxy server forwards the 180–Ringing response back to UA1.
9.

UA2 sends a 200–OK response, when the call is accepted by the user (for example, by picking up the handset), .
10.

The proxy server forwards the 200–OK response back to UA1.
11.

UA1 sends an ACK directly to UA2.

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Posted by sagitraz

Consider UAC , Proxy and UAS

1. (UAC -> Proxy) The UAC sends an Invite to the Proxy that includes a URI address for the UAS

1b.(Proxy -> UAS) The Proxy looks up the IP address of the UAS and sends INVITES to the UAS into the network

2. (Proxy -> UAC) The UAS does not respond immediately, so the Proxy sends back a 100 Trying saying that the UAS was found, registered, and the Invite was not rejected by the Proxy
3.
4. (Proxy -> UAC and UAS) Eventually, the UAS phone starts ringing. The UAC is notified of the progress with 180 ringing message
5. (Proxy -> UAC and UAS) As soon as the UAS goes off-hook, the UAC is notified with an 200 OK
6. . (UAC -> Proxy) and (Proxy -> UAS) The UAC acknowledges(ACK message) that it received the off-hook notification
7. As soon as the Ack reaches the UAS, the two can start communicating (RTP) The call can go on for as long as necessary
8. In this scenario, the UAS hangs up first and sends the Proxy a Bye. (If the UAC hung up first, the arrows would go the other direction.)
9. The UAS acknowledges with an OK, saying disconnect was complete

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Posted by george99