I don't know if this is possible in Asterisk, but if it is, I suggest that you include a checkbox when setting> up a route that says something like "this is an emergency route" - it would normally be a route used only for 911 (in North America) or similar emergency-only numbers elsewhere. If a route is> designated as the emergency route, it should have the> ability to take over a trunk if none are free for the call. Ideally, I would suggest that, for example, when 911 is dialed the system would first look for an available> trunk, but if none is found, it would first deliver a quick recorded announcement to all parties on the> needed trunk (something like a couple of distinctive> alerting tones, followed by "This line is needed for an emergency call - Goodbye" - it would then disconnect> the call, wait two seconds, seize the outgoing trunk> and dial the emergency number).>> This would primarily be useful for users that only have> one or two PSTN lines, but those are the only lines> that can send a 911 call to its proper location. The only time an emergency call should fail is if all> useable trunks are already in use for emergency calls> (if an explosion happens, everybody in the building> might attempt to call 911 at the same time, and you> don't want the first person who gets through to be cut> off by a second, third, fourth, etc. caller just as> they are giving the details of the emergency).>> If there is more than one candidate trunk, I would> suggest that the trunk to seize would be selected by this logic:
First choice: Trunk with call not yet completed (caller is listening to ringing or other call progress> tones). Second choice: Trunk with call in progress for 10> seconds or less (could make number of seconds> configurable) - if the call has JUST been established, the caller is probably still listening to an IVR or answering message or something like it. If more than> one trunk fits the criteria, then the one that's been connected the shortest amount of time. Third choice: The call that's been in progress the> longest - chances are more likely they've got their> business completed and are just shooting the bull, so to speak. My worry is that in cases where you only have one or two PSTN trunks and they are in use, a caller trying to> call 911 will get congestion. It's better to have a> dedicated emergency trunk, of course, but most small> businesses and nearly all homes will NOT have a dedicated emergency trunk.
