Direct Inward Dialing over Analog PSTN Possible?

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Direct Inward Dialing over Analog PSTN Possible?

Postby mtinker » Tue Nov 25, 2008 4:37 pm

Hi all,
I have a TDM800 connected to 6 analog FX0 Lines. Can/does the phone company send the dialed number information over analog lines? We have a couple of different 1800 numbers that all hunt the six lines, but I would like to handle the IVR for each 1800 number differently, and I would also like to setup a couple of extensions to have a direct inward dialed number. I know all this is possible over a T1, I was just hoping it would also be possible over analog line.
Thanks, for your help.
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Postby david55 » Wed Nov 26, 2008 5:10 am

Which country, which phone company, and which service product?

This is probably a question for the phone company, but someone may recognize the combination.
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Postby Raffles » Wed Nov 26, 2008 6:12 am

I'm guessing you are in the USA.

Anyway, you can't have DDI/DID presentation on analogue trunks (well, not that I am aware of) as there is no data signalling associated with them (like there is with ISDN).

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Postby mtinker » Wed Nov 26, 2008 6:23 am

Thanks for the replies. I am in the USA, using AT&T. I couldn't find much information about using DID over analog lines, but I did see that it was designed in the 1960's, so I figured it might have been in use over analog at some point in time.
Thanks again.
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Postby david55 » Wed Nov 26, 2008 6:25 am

It's certainly possible to signal both ways on analogue trunks. That is what happened historically. It is mainly done on tie line trunks, rather than central office ones, i.e. ones that don't use ringing voltage to start.

Whilst it is technically possible, I don't know if anyone does it for FXO and, if so, whether Asterisk supports their method, but the first thing to do is to find out from the PSTN operator exactly what service they provide.
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Postby ianplain » Wed Nov 26, 2008 6:47 am


Alog DID trunks were usually earth signalling and not loop, the DID was DTMF inband and was used extensively in the days before IDSN became available. Im guessing thats its withdrawn by now, The last system I had that used it was a science park in surrey UK. I think they finally swapped to ISDN in the early 2000-2001. In the end they were deliverd by a ISDN then a C-mux. Im sure they are still sites with Alog DID in the US as it very big there.

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I have direct inward over analog FXO

Postby fzied » Mon Dec 08, 2008 4:59 pm


I was able to do direct inward, but using freePBX, which is the standard on *NOW 1.5.

you have to create a "ZAP channel DID" and use it, that's it.

good luck.
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Postby newlinuxguy » Sun Dec 14, 2008 5:37 pm

In Canada we have a DID service on Analog. It's not used very often but it's available.

Ask your carrier if you can get "Ground Start" analog (as opposed to Loop Start which is a regular analog line.

The problem we have with these "Ground Start" in Canada is that they can only be used for Inbound Calls, there is no CallerID or any features.

Another option would be BRI (Basic rate interface) which is like a Mini PRI with 2 Voice Channels and 1 Data channel for signalling.

Or you could look at a SIP provider over the Internet.

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Postby leejor » Wed Dec 17, 2008 4:17 pm

DID trunks are quite common, both DTMF and even dial pulse. Our phone company has a lot of customers still using them. Many are migrating to PRI but there are a lot of older PBX's going strong. All the ones that I've seen are loop start.

A DID is basically a set of phone lines (FXS's) that your PBX sends back to the central office, you are sending the 48 volts towards the CO. You tell them how many digits you want to receive and you then send the call off to the extension. Usually companies require that you purchase a minimum of 4 trunks and you also have to get a range of PSTN numbers depending on how many extensions you want people to be able to reach directly.

One difference with a DID trunk is the ability to send a "wink" to the CO. DID trunks can be either immediate or wink start. With immediate the CO switch will seize the trunk towards you and send the digits, it assumes that there is a working connection on that trunk, usually dial pulse trunks are/were set this way as a default. As DTMF became available they came up with wink start. the original PBX's had a limited number of DTMF receivers as they were quite expensive. When a call came in on one of the DID trunks a receiver wasn't always available right away. So wink start was developed. When the DID trunk is seized by the CO the PBX will send a wink, a quick current reversal towards the CO to tell it to send the digits. If no wink is received then some CO switches will try the next trunk. DID trunks are also usually capable of sending a CO a reversal when the extension is answered, this will tell the CO switch when the extension has hung up and to disconnect the call if the caller hasn't yet hung up.

So, if you don't need wink start or the reversal on answer, then you could use a common FXS card back to the CO. All of this depends on what your local PSTN carrier requires.
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