Path: utzoo!telecom-request Date: Tue, 11 Jun 91 14:21:50 PDT From: "John R. Covert 11-Jun-1991 1710" Newsgroups: comp.dcom.telecom Subject: Re: Modification of Ringback Tone by Subscriber Apparatus Message-ID: Organization: TELECOM Digest Sender: Telecom@eecs.nwu.edu Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 11, Issue 450, Message 10 of 11 Lines: 52 > I think Tom Gray was right when he said if a PBX gives you a double > ring you're paying for the call. He's wrong, at least in the case of 416 392-7715 and most other cases. > I've asked three telecom CO people this question and all agreed -- the > last CO through which the call passed passes the ringing tone. That > is true even on DID connections. If you get a double ring from a PBX > it is because the PBX returned answer supervision and is supplying its > own ring. Kath, you're simply wrong. I happen to be an expert on DID trunks and PBXs. I either know more than the CO people you talked to, or you did not ask the right questions or understand their answers. DID trunks work as follows: CO seizes the trunk and sends digits to PBX. CO cuts voice path through to PBX. What the caller hears from now on, ring, busy, or recording, comes directly from the PBX. At some point, the PBX may or may not return answer supervision. Until the PBX does so, the call is free, regardless of what sounds come out of the PBX. For this reason, AT&T, on long distance circuits, does not cut a FORWARD voice path through until answer supervision is returned. It's pretty simple. If the CO were providing ring, busy, recordings, etc., you would have to have a protocol from the PBX to the C.O. that told the C.O. whether to send a ring or a busy signal. And there would be no way that you could get no charge recordings provided by the PBX that tell you that an extension isn't valid and give you the main number. There is no such protocol in use on DID trunks. The ONLY information a DID trunk can send to the C.O. is answer supervision. There is no way for a DID trunk to tell the C.O. to provide ringing tone or busy or other responses once the digits have been sent to the PBX. As I said in an earlier reply, I called 416 392-7715 and received a double ring. I did this from a line which provides a positive indication of answer supervision. There was a double ring, and there was no answer supervision. If you choose not to believe the experts, there's not much more I can say. john