Controlling polarity reversal ?

Discussion in '3CX Phone System - General' started by dsk, Dec 14, 2013.

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  1. dsk

    dsk

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    Hi
    I have been playing around, for a year or so, and my next step is to control an old payphone.

    When the other party answers, the polarity reverses. (if the ATA are configured for this).
    I you are calling some numbers who are supposed to be free of charge, the polarity does not change.
    So my first question: How may I control this. (directly connected to Callcentric, it works without reversal on some numbers).
    The next, and far less important issue are a possibility to send a billing signal. e.g. every 3rd minute. It may be a pulse of reversal, or a 16 kHz tone.

    Anyway, I have had great fun with mixing 3CX with mechanical exchange, and rotary phones.

    dsk
     
  2. leejor

    leejor Well-Known Member

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    What exactly are you trying to achieve? A reversal on every call?

    If you have a gateway connected to PSTN, and a reversal is send by your provider, then you have two options.
    A diode bridge will stop the gateway from seeing a reversal, but also ringing voltage (AC), so you have to bypass with a capacitor. Haven't played around with that because the ringer was always placed on the "outside" of the bridge. There may be some circuits on-line that you can build. Shouldn't be that difficult.

    The only other option is to set the gateway to ignore the reversal.

    This sounds very country/provider specific and would probably require you to construct a circuit to emulate the billing methods used by a central office. I suspect that it probably isn't something available commercially, but a search of the net might turn up something.
     
  3. dsk

    dsk

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    Thank you for the quick answer.

    The reverse polarity identification are equal for older systems in USA, Canada, and here in Norway among others.

    I want to use a PAP2T ATA and a pulse to tone converter for the old payphone. Caller Conn Polarity are set to reverse.
    1 When I call another subscriber at my 3cx the polarity should reverse on answer.
    2 When I call e.g. the extension chosen to be operator, it should never reverse polarity.

    The billing system is one of many country specific systems used, and are of less importance. Still if any option are available all ready, I do not have to make a circuit to solve this. (I know about several systems, 12 or 16 kHz signal, 50 Hz signal, DC signal to ground, short pulse of reversal, short interruption in power supply etc. )

    dsk
     
  4. leejor

    leejor Well-Known Member

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    I'm quite familiar with polarity reversal being used for business and coin phones. It was the "norm" in the step by step days but in many cases, depending on the make of central office switch/linecard, is available only when specified.

    It's one thing to simulate a reversal for all calls, but to prevent it from happening on some is much more difficult, especially if it does occur when calling another extension number.

    Two things I can think of...

    If you routed the operator call out of 3CX via a gateway (analogue) where no answer supervision is used, then back in. Don't forget that a 2102 can convert an "operator" number into anything else to sent on to 3CX.

    Use the FXS port on an SPA-3102 (if you have one to experiment with), using the internal dialplan, route regular extension calls to 3CX on the LINE 1 registered as an extension. Route operator calls out on the FXO port that is in turn connected to an ATA registered to 3CX. That ATA could be set to not send reversal when answered, preventing any supervision back to the FXO port on the 3102. I haven't tried it, and can't guarantee that it will work.
     
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