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).