[asterisk-users] question on softhangup

[Mitch Claborn]

Dave Platt provided the following answer to a similar question of mine 
last week.  I was trying to use SoftHangup() to prempt a DAHDI line for 
an emergency call.  Here is his reply.

That may be due to a common characteristic of PSTN lines (at least,
it's common here in the U.S.)

By design, most U.S. PSTN lines have a very asymmetrical response
to a physical hangup:

-  If the calling party hangs up, the call is terminated
    immediately.

-  If the called party hangs up, and the calling party does not,
    the line remains "live" for some time (typically around 30
    seconds, I believe).  If the called party goes off-hook again
    during this period, they can resume the call.

If I recall correctly, things were designed this way so that
the called party could say "Oh, hang on, I answered this call
in the bedroom and the stuff I need is in the living room",
hang up the extension phone, go to another room, pick up the
other phone and carry on with the call.

If that's what you're running into here - if the line you
are trying to SoftHangup() was handing an inbound call - then
there may be no good solution.  As far as I know, there is no
way to force an incoming PSTN call to release the line, other
than "go on-hook, and wait for 30 seconds to pass".

Several possible workarounds, roughly in order of increasing
complexity and decreasing reliability:

(1) Keep one of your PSTN lines reserved for emergency calls
     only;  remove it from your inbound hunt group and place
     it in a Dahdi line group of its own (or don't group it at
     all).

(2) Keep one of your PSTN lines reserved for *outbound* calls
     only;  you should be able to SoftHangup() an outbound call
     within a second or two.

(3) Figure out a way to check the PSTN lines that are in use
     at the time of an emergency - if they're all in use,
     somehow find one which was in use for an outbound call,
     and select it as the one to SoftHangup() and dial upon.

(4) If you must keep all of your PSTN lines in bidirectional
     use, you may have to *tell* the parties that the line is
     needed for an emergency call, and ask them to release the
     line.  Do a barge-in on the channel, play an alert sound,
     play a message saying "Emergency call in progress, please hang
     up this line immediately, play the alert sound again for
     a few seconds, SoftHangup(), Wait(2), and then try dialing.


Mitch

On 10/16/2012 08:59 PM, Jerry Geis wrote:
> How do I use softhangup through the AMI interface?
>
> I am using 1.4.43. Will softhangup hangup a DAHDI channel?
>
> I have found that "Action: Hangup" does not hangup a DAHDI channel only
> sip.
>
> Thanks,
>
> jerry
>
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