It is hard to understand how to route packets without a small introduction about the principles of SIP routing. Pay special attention to the difference between initial and sequential requests.
The requests are routed using some mechanisms in the OpenSIPS scripts; usually for inter-domain calls, we use a DNS server to discover the address for the destination, while intra-domain calls are often routed using the user location table. The replies are routed back based on the Via headers inserted during the path of the request. For stateful routing, the transaction is matched based on the branch parameter in the Via header.
See the following example, the address of the SIP proxy is
192.168.1.201:5060, the IP ...