Here is an example to show you third party call control. In this example Controller creates a call between Bob and Alice. The Controller first sends an INVITE to Bob. This INVITE has no session description. Bob's phone rings, and Bob answers. This results in a 200 OK that contains an offer (session description). The Controller needs to send its answer in the ACK. To obtain the answer, it sends the offer it got from Bob in an INVITE to Alice. Alice's phone rings. When Alice answers, the 200 OK contains the answer to this offer. The Controller sends an ACK to Alice, and then passes the answer to Bob in an ACK sent to Alice. Because the offer was generated by Bob, and the answer generated by Alice, the actual media session is between Bob and Alice.