TELECOM Digest OnLine - Sorted: Re: Supervision For DID Lines?

Re: Supervision For DID Lines?

John R. Levine (
15 Nov 2004 15:24:57 -0500

> A question I've been pondering for a while: When you call a DID
> number, at what point does the call start to be billed?

If things are set up correctly, when it's answered, the same as any
other call.

Classic DID switches the roles of the central office and the PBX. The
PBX provides battery on the line, the CO seizes the line and outpulses
the digits, the PBX provides ring tones and supervision, I presume by
revsersing line polarity. If it plays a recording but doesn't
supervise, you shouldn't be billed unless you have one of those lame
LD companies that bills after a fixed time rather than checking
supervision. In practice, this wasn't always set up correctly, and I
wouldn't be astonished if some of the recordings supervised, although
I would check for a LD company fixed time rule first.

These days, the preferred way to provide DID is over ISDN, BRI for low
volume or PRI for high volume, with the signalling happening on the
ISDN D channel, but the principle is the same.

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