If it's only the reps having this happens to AND they hear a true "BEEP" as opposed to more of a staticy click, check for the following:
(1) Check any stats you have to see if perhaps these calls are all disconnecting after the same call durration. Meaning, do all calls disconect after 5 minutes? (or some other amount of time )
(2) Is there anything on the phone LCD when this happen?
(3) Check the ACD ID's Agents COS, ALL Trunk COS and the COS of the instruments to be sure that "Call duration forced clear down timer" is set to Zero.
If you don't spot the cause there, we need to know the entire call progress path for the call: ie: DNIS rings to Auto Atten, then transfers to path, etc. We'll need a lot more detail. Hopefully you'll spot the problem in the COS.
Ralph