2 things to note, @Shar93 - "unknown calls" often come from numbers where the owner has asked for privacy, such as hospitals and health practices or clinics - such calls can often be genuine (and yes, they *could* make the call appear as if it was coming from a main number, but...!), and secondly, your phone needs to know the number for the block feature to work. A bit of a dilemma, wouldn't you say?
Scam callers almost always present a (often fictitious or random) number on your phone, and change that number frequently; Spam like double glazing or big call centres often mask their number so you cannot call them back. Catch-22! What can you do? Very little, alas.
You can block unknown or withheld numbers in your phone app settings.