A call has 2 legs: caller and callee. Each one takes one channel in asterisk, and if they are bridged together they count as 1 call.
When you have 1 call up between two extensions, you should see something like this in the asterisk cli when doing a "core show channels":
2 active channels
1 of 20 max active call ( 5.00% of capacity)
That is normal, the two channels are the legs of the call.
Now, if you have an inbound call that is not bridged to any extension and is busy in an application, like waiting on a queue, waiting for dtmf input, playing moh, leaving a voicemail, etc... as that particular channel is not bridged to any other, it will only count as one, and the output in show channels will look like this:
1 active channel
1 of 20 max active call ( 5.00% of capacity)
So, a bridged call takes 2 channels, while a non bridged call takes 1 channel.
Now, if you happen to use Local or Agent channel types, you will have to multiply that leg x 2.
