I would like to know if it is possible for asterisk to establish a three-way call on an analog line.

Typically, I would like asterisk to call 2 numbers, with the same telephonic line, and connect them together.

For a human, this is already possible on the line I'm using, by following those steps:

  • pick up and dial the 1st number, wait for it to answer,
  • press R (the hook flash) and compose the 2nd number, wait for answer,
  • press R and then 3 so everybody is now in the same conversation.

This is interesting here because the line I'm talking about can make international calls to many destinations without extra charge, so I can use it (as a relay) to call from one country (when travelling) to another without having to ruin myself.

Can asterisk reproduce these steps automatically, and do you know how?


Yes, this is possible with some dialplan scripting.

  • You would want to use the Dial() application to "pick up and dial the 1st number".

  • With a dumb analog line, you might not get any indication that the call was answered by the remote side. You can either have the first callee enter a DTMF digit when they pick up to indicate that it's time to call the next number or try to use call progress detection (callprogress=yes in chan_dahdi.conf). Call progress on analog lines isn't an exact science since it uses DSP techniques to try and determine when the line is ringing, or someone answers, etc..

  • You can use the Flash() dialplan application to send the hook flash.

  • You can use the SendDTMF() dialplan application to dial the 2nd number. Same problem with knowing when the line is answered by the remote side.

  • Thank you! Is there any tweaking needed for the Flash() duration? I've heard it can be different between countries. – Totor Apr 2 '13 at 13:22
  • Yes. What you want is most likely txflash in chan_dahdi.conf. – sruffell Apr 2 '13 at 17:18
  • I see no mention of txflash, but there is rxflash, is that what you meant? – Totor Apr 3 '13 at 22:13
  • Sorry. I meant just 'flash'. – sruffell Apr 4 '13 at 2:14

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