[Asterisk-Users] Random Behavior on Trunk Lines with TDM Card

[Andrew Kohlsmith]

On Wednesday 27 July 2005 11:51, Heath Bowlin wrote:
> We have implemented * in one of our branch offices and recently ran up
> against a very strange issue. On random occasions, when we would dial
> out using our trunk lines, we would get a message stating "you do not
> have to dial a 1 or 0 when calling this number" even if we didn't dial a
> 1 or 0 in the dial sequence at all. After much troubleshooting, we found
> users with similar issues that simply put a wait (w$EXTEN) in the dial
> sequence and fixed the problems. We did the same and it automagically
> fixed itself. This is great, of course, but I was wondering if someone
> out there could give me a more detailed explanation of why this works
> and why you have to implement this trick with some telecom providers and
> not with others. Verison provides our trunks at our branch office, but
> this issue has never arisen at our home office where we have SBC. We are
> running * v1.0 with a Digium 4 port FXO TDM card.

I am willing to bet that you dialled a number with '1' or '0' as the second or 
even third digit of the full number, and that your LEC is either overloaded 
and/or running older, slower technology.  

What I believe is happenning is that Asterisk is picking up the line and 
dialing, but the LEC switch isn't fast enough and only begins "listening" for 
DTMF after Asterisk's already played the first few digits.  This is well 
documented and the typical solution is to Dial(Zap/g1/w${EXTEN}) instead of 
just Dial(Zap/g1/${EXTEN}).  

The problem stems from a combination of the LEC switch being slow to begin 
detecting DTMF and from the Zaptel drivers not waiting for dialtone.  You may 
notice that this problem only occurs at certain times of the day and this is 
due to the LEC's call volume.  When you call during busy times it may take a 
few seconds for the LEC's switch to "get around" to your line.  Do you 
sometimes notice that you don't hear dialtone (with a normal phone hooked up 
to the line) for a second or two?

-A.