Callblock represents the start of an interesting new app category. Apple supports something called CallKit, similar to the content-blocking API that allows ad-blocking in Safari. CallKit allows you to build filtering rules to block calls by name and number.
The team that built Callblock spent most of their development time compiling a directory of telemarketers and setting up the infrastructure to keep the directory up to date.
So far, since I’ve had this installed, I have not gotten a single telemarketing call. Coincidence? Perhaps. But I’ll keep Callblock enabled, just in case.