How to detect outgoing call answer when using fxo channel

When I dial out using an fxo channel using a .call file, asterisk considers the call answered once the zap channel takes over. My settings for WaitforSilence() must account for the silence bewtween rings which is about 4,250 ms long (in united states). The pause is a bit long for humans which i think should be about 2 secs before a recording is played to them. This is for an appointent reminder feature, not telemarketing.

This is not a problem with iax or sip channels where true answer detection works. Unfortuneately, i’m stuck with fxo lines for now.

Is there any way to detect a true outgoing call answer with an fxo line?

not really. Unlike sip/iax/pri connections, pots does not provide answer supervision, that is any sort of indication that the called party has answered, other than the fact that ringing does not continue. The only theoretical way to do this is to know ring cadance and if there isn’t another ring, then you are answered. There is no call control signalling to indicate the line is answered.

The only way to not leave callers hanging there would be to play some kind of loop while listening… after the silencedetect hits then play their reminder.

If you are stuck with telco, maybe look into a fractional PRI line? You can get a PRI with maybe 6 or 12 channels active, which might suit your needs. Pricing is wierd on those however, depends on your area. Talk to your telco and any CLECs in your area and see what they ahve to say…

the only POTS hardware that i’ve used that does support/work with answer supervision is the Sangoma AFT A200 … as long as your telco provides it. then you can use “answeronpolarityswitch=yes” in zapata.conf.

ah, i wasn’t even aware such a thing existed. learn something new every day!

it caught me out the first time i used one. every call i made, Asterisk hung up as soon as it was answered because the telco provided a polarity reversal on answer and “hanguponpolarityswitch=yes” was set.

thanks for the tips! I thought it was an asterisk quirk, but it’s just another reason to ditch high cost ma bell.

Do you think that many pots providers have this polarity reversal on answer, and we just don’t know it because we are using TDM cards?