Some websites, when you register, ask for your phone number, and make it a required field. Some of them are smart enough to disallow 111-222-3333.
I enter: 718-479-7990, the number to the Rejection Line. If you haven't heard of it before, give it a ring when you need a laugh. And keep the number handy for those forms. Particularly handy when buying tickets for some event, to avoid telemarketing calls in the future.