on 26-08-2019 21:15
The only way is to block all unknown numbers, ie you can't be selective.
Some phones have that feature built in to the phone app whilst others will need an app such as Truecaller.
OnePlus 6 (O2 & Sfr), Z3 Tablet (Three UK), iPhone 8+ (EE)
on 26-08-2019 21:17
Which phone do you have?
It would normally be in your call settings and say something like 'block all unknown numbers'.