Tracing Phone Calls

Does anyone have knowledge of how phone calls could be traced in the 1970s (if this could be done at all)?

On old TV shows like Five-O, there is sometimes "call tracing" seen as a plot device, which usually fails because there is not enough time to complete the procedure.

I found this posting on some WWW site:

In the days of mechanical exchanges, all traces of a local call would vanish within a fraction of a second after hangup, as steppers would get reset to home position in preparation for another call. I don't know if any such exchanges are still in use in the USA; the last time I used one was in 1994. While such exchanges are rare if not non-existent, they established the convention that phone traces are performed at the Speed of Plot, and the convention has persisted even when the reason behind it didn't.

On Five-O, in order to >really< trace a call, you would have to have someone waiting at the phone company to do this. You can't just pick up a phone and call someone at the phone company and they can produce immediate results. (This almost always fails.) You would have to plan the trace ahead of time. Someone at the phone company would be in a back room at the phone company where there were all these racks of wires and switches, and could somehow track down each number of the call (or so it seems) ... if they had enough time.

In some H50 episodes, they are able to determine if a pay phone was used to make a call, but I think that applies when the call was international, because then there would be a record of that for long distance billing purposes.

I think they're right. I didn't get into communications/phone switching until the late 1980s where I joined the USAF. I know the response to any phone calls such as a bomb threat, is to still keep the person on the line as long as possible so a "trap and trace" - if the phone system allows that - can be done. Tracing isn't the problem, it's the "trap" aspect (something started in the 2000s) and I think it's a matter of either keeping the line connected or actually trapping the call. Fortunately, I never had to use that part of the system.

I know in the 1980s and 1990s, if we did receive a call, we were not to hang up our end. For whatever reason, I think the last number to call in could be figured out as long as you kept the line open.

I would say the closest Five-O got to reality is the stock sequence of Chin and the telephone guy going through the motions. They were in place ready to go but they also knew the number being called - the pay phone McGarrett was at - and they had to track down the other end.

Another one close to reality was in 6,000 Deadly Tickets. Again, knowing the number called into it became a matter of tracking down the other end.

Any other instance, I chalk it up to Five-O and HPD had the necessary system already in place. Was it reality? Maybe not but I would only know if I had an idea of the actual capabilities of the phone systems in place at that time.

These days if you get a call where someone is threatening to murder you or something equally horrible, if you hang up and dial *57, that supposedly will keep a record of the call, including a trace back to wherever it came from. At least that is the way it works with my phone company. I have never had an opportunity to test this out, though. After you do this, you are supposed to call the cops with details of the call and they can they work with the phone company to track down where the call was coming from. I dunno if that would work with some of these robocalls and other scams, though.

A friend of mine had this comment on this issue:

Thru the 70’s, every phone had to have its own pair of wires. That pair ran all the way from the phone company to the phone in your living room.

To trace a call, they would indeed have to have a live conversation taking place so they could sense the current in the wires and follow it through the switches and to the exact pair going out to your house. Worse, even once they found the pair, they’d not immediately know what house it went to; they’d need to have it marked or look it up in a table or book because thousands & thousands of wires went out from the switching station to neighborhoods, in huge bundles as thick as your thigh.

I’m sure there are websites somewhere that describe it. But it faded out with the advent of “multiplexing” which is using a single wire to carry multiple lines by using math to combine their signals together on one and and decrypt it on the other. What this also started allowing is knowing which lines were going into each multiplexer, so tracing became easier/faster.

This is correct.

The ability for the phone company to trace calls was always overstated in 1970s TV crime dramas.