Call blocking / labeling currently happens at the recipient’s carrier. Carriers watch for how many calls they get concurrently or overall from a unique phone number, if its a disconnected number, and are beginning to use caller ID reputation registries. Additionally the end user may also be using various smartphone apps that crowdsource reports of nuisance calls.
Even if you don't telemarket, scammers can hijack your number and ruin your business phone numbers reputation. See our advice below for both telemarketers and business owners.