Several telephone operators offer a call screening service, this reduces the chances of a scammer or a sales person from being able to contact you, provided you use the service correctly.
It is often provided as a free service but it can block essential services.
You can also purchase a truecall box from truecall.co.uk which provides a similar service in your own home. However, you can take advantage of some of the more unique features of truecall that allow more fine tuning such as what happens when a caller makes a call, whether you need them to call someone else for access, or that you wish to record the call.
On mobiles, it is often harder to use a call screening service. On an iPhone, you can use Do Not Disturb, coupled with Allow Calls From contacts in your contact list. Unfortunately, the Android Do Not Disturb system is far more complicated and depends on which version of Android you are using.