The problem with phone number based filtering at least in the UK is that many large buisnesses and institutions have their phone systems set up to withold numbers for all outgoing calls. So you have no way to tell the difference between a call from someone at work, a call from your kids school, a call from the hospital and a telemarketer.
The bigger problem is that even if the number is there, it's usually a fake number or a different number every time. Blacklist doesn't work when telemarketers and scammers have phone banks with thousands of outgoing lines. The only real solution would be whitelists but then any unknown caller (like a school line you didn't know about) can't call you.