2.4.27-2-386 #1 Mon May 16 16:47:51 JST 2005 i686 GNU/Linux
asterisk 1:1.0.7.dfsg.1-2
zaptel 1:1.0.7-4.1
We experience internal channel ‘bleed’ where internal users
are able to overhear each others conversations during their own
calls , but the remote party cannot hear this effect.
Any pointers on this would be greatly appreciated as I’m a real
noob to asterisk and voip. 
it’s called crosstalk, and one thing you might consider would be to update asterisk to the most recent 1.0 build if you want to stay on that branch, or the most recent 1.2 build if you want to move to the new branch.
There is very little information in your original post, so you’ll need to tell us more about your config, specifically how your users are connecting to asterisk, and how asterisk is connecting to the outside world.
If you are using analog lines internally, that would be my guess as to where you are getting the crosstalk.