There already is a number - 101. It's illegal for dogs to be dangerously out of control in any place. The problem is by the time they're dangerously out of control it's too late.
You couldn't have a system whereby anybody could report a dog "they believe to be dangerous" as it's so subjective. Lots of people who are scared of dogs believe any big dog might be dangerous, or any dog off lead. It could only work if the dog was actually behaving dangerously, otherwise the dog/owner would have nothing to answer for. So we need to act well before it gets to the stage where any dog might be acting dangerously.
I'm fully behind requiring a licence to own a dog, and if I'd go further. I would advocate compulsory basic training, which must include training of the owner, not just the dog. There must be a compulsory element on understanding canine communication and body language.
Insurance should also be compulsory.
Every element of the dog industry should be regulated - from breeders to dog walkers, groomers, trainers, etc. Dogs are not just commodities and dog ownership should be taken far more seriously.