It is important to understand the basic processing of a SIP proxy according to RFC3261. Without this understanding it will be very difficult to configure a Proxy server.

Each proxy will take routing decisions, modifying the request before sending it to the next element. The responses will be routed over the same set of proxies traversed by the original request in the reverse order.

A SIP proxy can operate in stateless or stateful mode. When a SIP proxy works as a simple SIP packet forwarder, it forwards the packets to a single element determined by the request. A proxy working in stateless mode discards any information about the message after the message has been forwarded. This characteristic limits the failure treatment and billing.

When OpenSER knows that the message 200 OK corresponds to a specific INVITE we say that it is working in stateful mode. This means simply that you can now manage the response in an onreply_route() block. With stateless processing each...