Until this service becomes so popular that the robocallers start deliberately working around it, I think you may only need a 1-digit PIN. I've had a service fro

Until this service becomes so popular that the robocallers start deliberately working around it, I think you may only need a 1-digit PIN. I've had a service from the phone company on my landline for decades that just makes an unknown caller dial a 1 before my phone will ring, and it continues to nearly eliminate spam calls. So the 1-digit PIN can just be a "1".