Can Asterisk detect if an outgoing call is answered?

Good evening, all.

This is a problem I had a couple of years ago and never did solve it with the hardware I had.

Somebody calls in and presses 1 for Jack. Jack’s internal extension rings three or four times, then Asterisk says “Jack’s AWOL; let me transfer you to his cell” and it makes an outbound call to Jack’s cell phone.

All of this works fine.

However, if Jack doesn’t answer his cell phone, I want Asterisk to pick the call back up and redirect it to Jack’s internal voicemail.

I never did get that to work with the Digium TDM400 card because that card is incapable of detecting when a call to the outside is answered.

When I ran into this problem in the past, some of you here told me that there is some kind of signal that the telco can provide (I think it has to do with reversing the voltage polarity on the POTS line) that signals that a call has been answered. The TDM400 cannot make any use of this, but I was told by someone on this forum that the Sangoma A200 card can.

I’ve been going back and forth with Sangoma tech support, had a bit of difficulty making him understand exactly what it was I wanted, and he said the A200 cannot do that.

So, I’m back to square one. Is there any hardware out there that will support this capability? If I find some, then what do I need to say to the telco to enable whatever service it is that provides this capability?



you wont get a pots line to do this, or at least the effort involved will be too great.

Use a SIP or IAX truk as these have answer supervision.

Also implement a dialplan that makes the called mobile accept the call ie dial a number to accept that way if the call goes to the mobile voicemail it will still get pulled back. and only ring the mobile for less than its call forward to voicemail.

Do these and you should be fine.


POTS lines can have answer supervision, and it is typically implemented by reversing polarity. However whether it is available here depends on the service provider.

Good morning, David.

My question is, what POTS interface hardware can take advantage of this? The Digium TDM400 can’t do it, and I was told a couple of years ago on this forum that the Sangoma A200 can… however the Sangoma tech support person says it can’t.


The best thing to do if you want to go down this route is to put a DVM or scope on the line and make a call this will show you polarity of the line and when it switches if it does, The problem is some suppliers will reverse polarity on CSA some on commencement of far end ringing.


This Asterisk Issue seems to imply that TDM 400s can support line reversal detection and Asterisk is now capable of using that for answer supervision: