Says who? If you're answering all calls either manually, or automatically, your phone is likely to keep ringing since those are the kinds they like. It won't happen overnight, but if you're able to deny any kind of answer on unknown numbers, your number might start being added to pests' lists of ones to quit wasting time with. It requires patience, being able to miss calls, and can be nerve-racking, however, it's how I stopped them when my landline was swamped daily. You just have to let the phone ring clear through.