To start with, you should have the CO ring delay timer set for up to 6 seconds in order for the caller Id to be present before the phones ring or the AA answers. Then, the CO Preset Forward timer controls how long the attendant rings before forwarding to the AA pilot number. I prefer creating and using hunt group 450 instead of VM group 440.
Now if you set the key action in the AA "0" to either "0" or 100, then the stations No-Answer timer will be used to send the unanswered call to voice mail 440. However, attendant stations don't always send their own ID to the voice mail and the caller could end up back at the main prompt.
The earlier flash VM didn't have a good time out option. The greeting would play 3x and then transfer to "0"