I haven't used a landline in years now, but when I was a kid it worked that way: one end hanging up wouldn't disconnect the circuit, and if the other end took the phone off-hook shortly afterwards it'd still be connected.
This would sometimes wind up being the deciding factor in the eternal battle of "which sibling gets to use the phone"
In my memory it never relied on both parties hanging up here in New Zealand, but on a trip to the UK as as child I noticed that the phone line disconnecting relied on both parties hanging up. I'm 31.
This would sometimes wind up being the deciding factor in the eternal battle of "which sibling gets to use the phone"