asterisk users - Jul 2007 - Queue to outgoing Zap channels when congested

Hi All,

Does anyone have an example dial-plan and/or a way to use the queues to 
access outgoing calls on Zap channels when all the channels are congested?

e.g. I have 20 users, who will be accessing Zap/g1  .. which has 3 channels

I have another 5 users who will be accessing Zap/g2 .. which has only 1 
channel.

The outgoing numbers are dialled directly, and put through categorised 
extensions to filter out / block unauthorised international, 
national/mobile, local, toll-free and emergency access, based on the 
users' assigned SIP context.

So if <3 channels of Zap/g1 are used, calls should be put straight 
through to connect the call.

If there are >=3 channels used, then calls should be queued, 
indefinately, until their turn comes, and then the number dialled and 
the line handed over to them.

Thanks,

bu