Unfortunately there's a scam for that,apparently. The scam caller tells the victim to call their bank, but doesn't hang up. The victim hangs up, calls the bank, the scammer is still on the line.

If you're concerned about that, wait a minute. I can't find any discussion of the scam online, but I would hazard a guess that this isn't really an attack vector these days.

It happened to an old lady in switzerland recently - she didn't hang up, the scammer just got quiet after saying he would hang up so she could call the police, and then had someone else answer the lady when she assumed she was talking to the police.

how is this allowed to be a thing?

The telco's themselves should be held legally responsible for the fraud in these cases.

If I hang up a phone, I expect that to be the end of it, period. Yes, that includes landlines.

..and for those who might not be comfortable trying to find this information on the internet, it's almost always listed on your [credit/debit/atm/etc] card itself.