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Asterisk, Digium TDM400 FXO, & Phantom Rings

Posted on 2006-06-12
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Last Modified: 2008-02-01
We have set up a customer with an Asterisk @ Home 2.8 system (went in before Trixbox was out). It has four BT lines (we're in the UK) connected to four FXO ports on a Digium TDM400B card. The only advice/help I have had was to add OPERMODE=UK which I did - and it made the problem worse.

The problem is that at the end of just about every call, the system acts as though there is another call coming in. I believe this is caused by a battery drop or polarity reversal on the line to signal call disconnect being detected as a ring signal due to too little debounce.

How do I fix this, it's driving the client mad !
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Question by:FurnessSupport
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feptias earned 1500 total points
ID: 16894713
I don't use @ Home so this info might not be relevant, but I noticed the following in my sample zapata.conf file:

; In some countries, a polarity reversal is used to signal the disconnect of a
; phone line.  If the hanguponpolarityswitch option is selected, the call will
; be considered "hung up" on a polarity reversal.
;
;hanguponpolarityswitch=yes

There is also some choice about signalling on analogue lines. This is also copied from zapata.conf:
; fxs_ls:         FXS (Loop Start)
; fxs_gs:         FXS (Ground Start)
; fxs_ks:         FXS (Kewl Start)

I use Kewl Start for an FXO line in UK and it doesn't give the problem you described. However my analogue line might not use exactly the same signalling as yours. Also, my system doesn't detect any hangup signal from the exchange so the line is left open until the local extension clears the call. Not ideal, but better than phantom ringing.
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by:FurnessSupport
ID: 16895155
Thanks for that, having the hanguponpolarityswitch keyword would have probably been enough to find what i was looking for with Google. As it is, I've just had an email from Digium (after I'd given up on waiting for an answer) to tell me I need :

answeronpolarityswitch=no

I've set it on the cusotmers system and it seems to work - just waiting to see if it causes any other unexpected problems.

Now, to find somewhere to put the information so that other might find it in future !
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LVL 19

Expert Comment

by:feptias
ID: 16895244
I saw the other parameter, answeronpolarityswitch, but I just assumed it would take a default of "no". It is a bit surprising that the default is "yes" because battery reversal is not commonly used to signal ringing that I'm aware of.

I'm glad you've got a solution and thanks for the points/feedback.
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