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.