In any case you should never answer a call coming from a number not in your contacts list. If it is a genuine call from someone not in your contact list they will leave a message. If the message is "hello, it's about that car accident you had, please call us on 03495837285" then obviously you don't call them back. If they are a marketing company they will not leave a message, because in fact it was a computer calling you, not a person.
I worked in a call centre updating the computers. A Big computer in the corner of the centre was dialling sequential numbers. If any number was answered, a light would flash up at a staff's desk. She would press a button and instantly all the called person's details would appear on her computer screen, even before she had said hello this is xyz bank. It was a legitimate mainstream bank, they were canvassing for new customers offering £100 to open an account. Not asking the customer for money.
I asked the team leader how it works.
1) The computer dials a random number. If the computer detects dial tone the number goes into a spreadsheet in Column A. Number is genuine.
If you pick up, there is a 2 second silence before they speak to you. This is a second way, in addition to the fact that the caller is not on your contacts list that you can recognise an unwanted caller.
2) If the call is answered, the number goes into Column B. Number is genuine and answered. Even if it is immediately shut down with nothing said.
3) If the called person actually talks, even if it is to say go forth and multiply, the number goes in Column C
4) If the called person talks sense and sounds interested the number goes in Column D
5) If the called person actually buys something or accepts the bank's special offer the number goes in Column E
Then the bank sells the data to marketing companies, the price increasing as the number goes from A towards E.
So the way to avoid receiving marketing and scam calls is never to answer a call from a source not in your contacts list. If it's genuine they will leave a message. But don't cal back if it sounds iffy OR if your phone just rings once then cuts off. That is part of the scam, to make you call back. The number might be a premium £5 a minute or part thereof starting from ring tone start, and looking like a regular 0793....number on your display.