If you are sure this is a 5000 and not a 3300 then it sounds like you are doing a CONFERENCE, not a transfer (the 3300 can do it this way)... You cannot just drop out of conference and leave the other parties connected, if you are doing a transfer you should not be able to get all three in the call the same time...
It should go like this:
Well on the first call, the agent presses TRANSFER, then 8+<number>, when the other party answers they can announce the call and then hang up and the initial party should be talking to the destination. At this time the Unsupervised CO system timer kicks ina nd will drop the call when it expires. At no time should be agent, caller, and destination be able to all talk at the same time.