Indeed, there is too little information about your setup and issue.
The key question is WHO and HOW is sending you these service messages.
When ISP tells you something like “The subscriber is not available now”, this message is usually sent in “183 Session progress” response, whereas when a called party answers, “200 OK” response is generated, and this response triggers charging.
However, if a remote party has an answering machine, it may automatically connect to the line, thus generating “200 OK” in response, which would not be distinguishable by an ISP in terms of signalling.
Answering machine detection feature might help you disconnect calls faster in the situation described above, but (as said) we need more info to clarify the entire scenario.