As to what the FO button does, it cuts off whatever call the number you were dialing is on and connects your call instead. So you had better be sure your call is is important enough to potentially cut off a General Officer.
At least that's how it was explained to me back in 2003. Never had occasion to use the priority modes.
It's more that on systems with a circuit switched network there are typically fewer outgoing/incoming lines at a location then there were telephones, so you can get in a situation where a choke point in circuits can mean that you can't make a call because they're all busy. FO will force hang up other calls to make sure yours can go through. That's very likely to hang up other, unrelated calls in order to get through, rather than the person you're calling (it will hang up their call too if they're currently on a call though).
In the late 90s I was with a group of people and we were trying to make a call. The line was busy, had been for an hour. We assumed it was the teenage daughter on the phone. One in the group took the phone, dialed a lot of numbers, then asked for the number we were trying to call. He punched it in and handed the phone back. The call went through and the confused teenage daughter answered - confessed because her call was interrupted. Everyone in the room was amazed and he went back to reading as if nothing happened.
I don't know what went on, but I can only imagine it was related to FO. My thought was he punched in to somewhere that would let him use a non normal button by way of numeric entry and connected the call - but I don't have a clue.
yes, it sets your priority for throwing others off of interswitch trunks and keeps others from throwing you off, as i understand.
traditional phone network would give a fast busy or an "all circuits are busy" message if all lines between two switches are busy. the special buttons let you assign priority for your use of those lines. F0 is just max priority.
...and now imagine how likely "all circuits are busy" would be in a network were half of the infrastructure was freshly nuked. A dedicated button for "give me a line, I'm the president!" doesn't seem that far fetched anymore. Who knows, if they spend enough time pondering communications availability in such dire circumstances someone might invent an internet!
As to what the FO button does, it cuts off whatever call the number you were dialing is on and connects your call instead. So you had better be sure your call is is important enough to potentially cut off a General Officer.
At least that's how it was explained to me back in 2003. Never had occasion to use the priority modes.