Dialplan conundrum - Stopping bridged ZAP lines

Hi All,
Am running asterisk v1.4.2 with around 20 Grandstream Budgetone IP phones through 5 zap lines. I had enabled the TRANSFER features on the phones, but then one “curious” user demonstrated to me how he could blind transfer a call to an outgoing line (i.e. bridging together two zap lines), leaving himself out of the loop. (Yes, I do have canreinvite set to yes)

Any thoughts on how to prevent this? The only approachI can think of is to disable the call features on the phones, and then use the transfer capabilities in the features.conf file combined with the TRANSFER_CONTEXT variable somehow to limit transfering to internal calls only.

If anyone knows of a better/more elegant method (or indeed has an example of my guessed direction), I would certainly be interested!! It would be nice if the users could still use the big shiny TRANSFER button…

All the best,

Why do you want to disable the ability for users to transfer out a zap channel? I see it as a fairly common need. As an example lets say an important sales call comes in for a sales person but he is at lunch, why not transfer the caller to his cell phone? There is no ideal way to accomplish what you want to do, the config you described is probably the only way. BTW, canrenvite configures the ability of a sip device to make direct contact with another device, it does not relate to transfers.

I dont have the answer to the original question but the DANGER in connecting to ZAP lines together if they are loop start without disconnect supervision is you end up with 2 trunks on your system Locked high and wet forever.

Thanks for your replies folks. Agreed that I certainly can see when bridging zap lines would be useful (though the users are graduate students, so should just happy to have phones at all!).

However, Christopher hit the nail on the head re disconnect supervison, which we have been unable to get working after three months of trying. Bridging the ZAP is not the end of the world - I have an absolute timeout set for 1 hour for cases just like these - however with oly 5 lines, taking out 2 for an hour dramatically increases the chances of blocking…

Maybe it’s time to revisit disconnect supervision - if anyone has any experience getting a TDM 400 working behind Nortel Meridian switches, I’d be very keen to here from them!