I think, when an answering machine or voice mail picks up, it some how clicks/chimes/gives some tone/or hesitates before the line is actually picked up/dont know, but is somehow different from when a person picks up a phone. SOME computer dailing machine can recognize this an hangup automatically then. Again read my disclaimer, not sure

On a very similar note, that telemarketers zapper thingy that supposedly blocks and removes your number from telemarketers databases automatically imitates the answering machines click/tone/chime/whatever to the computers hangup then. Though this doesnt remove you from their list like advertised. I think I saw this on some show testing "As Seen On TV" products and worked with an actual telemarketing firm to test this out.

Disclaimer #2: Results may vary

Varikin