AFAIK that is impossible. OpenSer accounting in based in transactions, not in single messages. A transaction involves a reply (or various).
BTW, how is possible "you can't need to wait for answer from other side"???
First of all - sorry for the mistake. My knowledge of English is rather poor.
We take accounting information (Acct-Session-Time) from Cisco CS2800 which provides us very reliable info about duration of calls. But before call will be established there is some complex logic with rewriting of Request-URI in multiple steps. We need to track all these steps for our internal purposes.
We could do it in a different ways and one of them we consider is to generate Start-messages (or some other message) for each INVITE passed though OpenSER exactly then INVITE is arrived.