We want to have a monitoring system that places phone calls to our on-call system administrators, and we're having trouble figuring out how this can be done cheaply and easily. We figured we can write a script that interacts with a VOIP-type API to dial out. The trick is we want to have the script accept a key press by the person on the other end of the phone as an acknowledgment. If there is no key press (no answer, went to voicemail), then the script will escalate to the next person in the tree.
We can handle the escalation part. What we are looking for is how to have the script listen for a key press by the remote user. We don't care what key they press, just any key press will do. Is this possible with any of the known API's out there without having to implement a full PBX?