Obtaining number of callers in queue

Running asterisk 2.5
I wish to add a feature so that our internal agents can type for example : 5000*** and it will return the number of callers waiting in a QUEUE.

So, if there were 5 people waiting in queue 5000, by then picking up the phone and dialing 5000***, it will announce that 5 people are waiting in the queue.

I would prefer not to utilize external software to achieve this. Is there a Asterisk function that will achieve this? I do not readily see any functions that ties to this kind of information.