When i dial 888 i get prompted to put in a channel, i dont know what to put in there. However, anything I try putting in disconnects the call?? How can i barge a call in a simple way?

I assume you are aware of that this works only with Analog Lines terminated to your server (DAHDI channels or ZAP channels).

If you have such channels, and there is a call in progress, you can dial 888 and enter 1# for channel 1 and so on to just listen to the call, I am pretty sure they will not hear you if you speak. Obviously, you need to know which Zap channel the call is on if you have more than one.