DAHDI and Centrex


We are having a lot of trouble with the Centrex dialing with our FreePBX. The Centrex solution supports 4 digit dialing as well as 10 digit dialing and requires a 9.

We have a lot of problems where when dialing out, the line says wrong number, or number invalid.

I have the trunks set to route 9XXXXXXXXXX to go to DADHI, but it’s returning that the line is busy or number invalid.

Any thoughts on what I could be missing?

Technically you need to wait a little for the centrex dialtone after pressing 9. That can be done by adding D(9w) , more w’s if necessary they are 500ms each, to the dial options for the trunk, unfortunately that will break any centrex extension dialing you do, so you will have to make two “pseudo” dahdi trunks to suit XXXX and NXXNXXXXXX dial patterns. It is then your decision to require 9 before external calls or not by your users.

That’s a good point. I didn’t even think about it.

So my dial pattern would be?


Pound Sign DWWXXXX

since we dial a pound to call station to station.

We have old analog phones so the only way to tell the lines apart with the hardware we have is to dial 9 to activate the DADHI line or 1 to activate the VOIP line.

I am not sure that the dialpattern supports that, I have always done it at the trunk level

I found it and set up 9wwww.

About 75% of the calls complete. It seems about 1 in every 5 to 6 calls returns “This number is unavailable.”

If I hang up and dial again, it completes successfully.

But I need to figure out why because if the call fails, the people assume the entire system is down and panic.

You might want to set


in your dahdi configuration, that might need restarting dahdi/asterisk not just reloading it

Okie. I’m not seeing that setting, I put in a paid request for that.


It sounds like dicko wants you to set “Other Global Dahdi Settings” in DAHDI Configuration, Global Settings.

You can set additional settings that way (for example I have relaxdtmf = yes there).

Good luck and have a nice day!


Centrex equipment is very old, they don’t need relaxdtmf , because it always increases the possibility of “talk-off”, they expect humans (aka 2500 like sets and 1A2 racks locally) and not machines to do the dialing, the 80ms dtmf signal often outpaces old Nortel and AT&T switches in the US which are mostly used to provide Centrex service. My suggestions are to better duplicate such human behavior

Oh ok. I will do that.

Another thing I tried was doing w9w in the trunk and that seemed to help a lot.