SIP. How to recognize transfer to voicemail?

Question

Answering machine

Hello!
The essence of the problem: for the work of the contact center, we use Asterisk-1.8.21.0 together with ViciDial2.10-448a. Telephony SIP. Calls are made automatically by the system and reach the operators only when the answer of the called party is recorded. Recently, the situation has been greatly complicated by the voice mail of mobile operators.
Our system, having heard something from the category "Ab-t is not available, you can leave a message after the signal ...", detects this as an answer and transfers the call to the operator, he takes the call, but no one, of course, answers his unsuccessful "hello" , will not answer. Now we are looking for a solution how to detect such auto-replies and stop them in the bud.
Options:
1. At the signaling level, trace such a call is identical to a normal call with a real person answer.
2. A solution like "press the button to talk to the operator" is not considered.
3. Rtptimeout is not considered (it makes no sense to detect silence).
4. The Auto Machine Detection function on ViciDial gives a heavy load on the system and low accuracy.
5. It makes sense to compare like standard samples like "leave a message" with what the system hears in the channel and decide who did answer us. This approach is implemented in Oktell Antirobot.
What solutions are possible???

Answer 1

I can offer you to limit the time of the call, as a blunt option. For example, try to reach the subscriber for no more than 30 seconds.