Don't Hang up the Phone

I suppose so they could trace the call, to get a handle on where he'd been. But, who knows - there's a lot about this movie that doesn't make sense.

If he hung up the phone and another call came in it would wipe out the information in the switch. Remember, back then, people could CALL payphones. You'd see people standing around waiting for a call to come in. By requesting that he leave the phone off the hook, there is no risk of another call coming in to wipe the info out of the switch.

So the Network Ops Center at WTC (BUILDING 7) could trace the call to the EXACT payphone. AT&T CN&A only shows that it's a payphone on lets say Main and Pearl. But which payphone as there are 4 there. A dangling handset pretty much eliminates any guess work for the damage control team first responders.