T-Lib-Initiated MakeCall INVITE
RFC 3725 is a configuration flag that can be set to 2 (the default, which causes the T-Lib-Initiated MakeCall re-INVITE call scenario to be used) or 1. When the parameter is set to 1, the call flow will be as given below. Notice in this case that according to the RFC (best current practices for third-party call control) UA1 does not get an ACK until UA2 has responded with 200 OK. In this case, SIP Server does not send a re-INVITE to UA1.
Note: Usually, the re-INVITE call flow is the preferred use case. This is the default behavior of SIP Server, unless the RFC3725 parameter is set to 1.