Hi there!
It is a great feature.
In the past I implemented a similar app for Android, and it has features like white/bllack list with numbers or prefix you allow to receive calls or block, and a call recorder too. It was discontinue due to Google limit access to Telephony API each version of Android after 4.4 KitKat so I gave up.
You could create lists using wildcards like:
+551134* Block
*5506 Block
+5516 Allow
....
Blist was one of my first Android Apps, so there will be a lot of "worst practices" 
but if you interested on look:
https://bitbucket.org/leonardoroese/blist/src/master/
Integrated call recorder could also be nice but is not allowed ind some countries.
Nice weekend for All.
Leo