That's not quite correct. You can register a number of different phone numbers. When I set mine up the first time, I was in a hotel in Los Angeles, so I had it ring the hotel room phone (I have no cell). I now have it set to my home number, and the hotel room phone number is no longer a registered phone.

However, when you register a phone, it requires you to validate that you own the phone, by ringing it up and having you enter a code, so you can't just register with a random number.