specifically, in ye olde days there was a significant period between the called party putting their phone down and the call ending. I presume it's a hangover from the days before cordless phones, where you might like to answer a call at one phone and take the conversation elsewhere. The call would still end but it'd be quite a wait. This was only ever the case for "analogue" landlines, anyone using ISDN or VoIP or as you say mobile would not be affected.That's less of an issue these days. In the past, only the calling party could end the call. But BT changed this a few years back and either party can now end the call. And i that's always been the case for mobiles.
As that ended up being used by scammers to pretend you're making a phone call to the real organisation (they'd play dialtone and ringing tone) BT eventually took it down to a second or two.