Had a look on the TPS website. Seems you're on a sticky wicket if you phone someone who's registered with them. You can get numbers checked for £50/month (500 numbers - after that it's charged per number)
Snags:
You have to register, and then you're charged £50/month whether you use the service or not.
It only covers you to check 500 numbers. If the first 499 you check are registered with TPS you only get one you can actually call
I'll stick with leaflets and doorknocking
Till they ban that in my area, anyway :