[Asterisk-Users] Freak incidents, who's to blame?

[John Novack]

Ryan Courtnage wrote:

> Hello all,
>
> Everyone has probably experienced this at some point in the past:
> You pick up your analog phone.  Rather than hearing dialtone, you are  
> connected with someone who has just called you.  Neither you nor them  
> heard a ring.
>
> Maybe it's just me, but it seems these "freak incidents" would occur  
> more frequently years ago, than now.
>
> I've now experienced this a couple of times with an * system (TDM400p  
> - quad FXO):
> A SIP exten dials digits which are answered by a Zap trunk.  As soon  
> as Zap answers, the SIP extension is connected with an inbound (PSTN)  
> caller (who was expecting to hear an IVR).
>
> My questions are:  Who's to blame (telco, tdm card, * config,  
> gremlins)?  Is this avoidable?
>
> Thanks
> Ryan

Really no one is to "blame"
This is known as Glare, or a head on ( collision )
Take a basic Telephony course before attempting to become a telecom 
engineer.

Back in the "good old days" a PBX would have  analog trunks that were 
ground start, and tip was open when idle. The PBX would have an 
interface that knew this, and  if it found ground on the tip would move 
to the next outgoing trunk. Ground was applied to the trunk by the CO 
BEFORE it was rung. The ring signal would then  start whatever sequence 
in the PBX was necessary to answer the call.
I gather that the FXO card doesn't support a ground start trunk at all, 
being nothing more than a modem given a new lease on life.

Something other than a X100 or TDM400 is needed, and analog Ground start 
trunks from the Telco are called for for proper two way operation.
GS trunks are certainly still available in the US and probably elsewhere.
If you need enough trunks, perhaps a PRI instead?

John Novack