My method is as follows:

- I don't have a landline phone

- If someone calls my mobile and they aren't in my address book, I just don't answer.

- If that number is withheld I especially don't answer

- If they leave a message I will listen to it and respond if necessary

- If the number is not whitheld but not in my phone book, I google it, and if it is a legitimate business, it will usually show a match on the company website. If I need to I'll call them back e.g. today I had the estate agent ring, but I didn't have their number saved so I didn't answer. I rang them back once I had googled their number