In Cast Iron version 4, you can call from orchestration to orchestration using a "local" call mechanism. This removes the need to communicate via web services. To do this, make sure that both ends of the call are configured as "local".

One caveat found so far - if the called service doesn't have a "send response" step, then the calling orchestration seems to just stop after the call. The calling orchestration errored, but there was no error in the log. In this case, we actually didn't care about the result and just wanted to return as soon as possible, so I added a "send response" straight after the "receive service" starter step. This sorted the problem out.