When my Dad had a nuisance call from someone claiming to be from his bank....he was advised by the bank itself to unplug the phone at the wall for 2 mins as these scammers have a way of staying online and collecting info from your outbound calls...don't ask me how?
Generally...if it is a scam or an automated call...they won't go onto answerphone ..
I know that if it's a landline call, both sides need to hang-up to drop the call properly. The bank's advice to you father was correct in so far as I've experienced this myself several times: as soon as I realized I was on the receiving end of an automated call, I put the phone down. After a few seconds I picked it up again to check I was now getting the dial tone, but instead I could still hear the recorded message! I think I disconnected the phone fully on one occasion, but usually I just leave it down for 5-10 minutes - although make sure you can hear the dial tone before using again!
There's a known scam where the conman will call you pretending to be a bank or whatever, tell you there's a problem with your account and advise you to end the call and phone your local branch. However while you hang-up, they stay online! When you then try to call your branch, they're still on the call: they can fake a ringtone, and then pretend to be bank staff on the other end of the line... In theory, this no longer works on modern phone networks (and it won't work at all on a mobile phone, which connects to the public network in a different way), but it's still best to be careful...
See "The 'hang-up' phone scam" here:
Common phone scams and how to avoid them - Which?