Because people will still try to dial them... Possibly even dialing all the missing digits in an attempt to find the 'real' one, even though it doesn't exist.

Because people will still try to dial them... Possibly even dialing all the missing digits in an attempt to find the 'real' one, even though it doesn't exist. If you start it with 555, then you don't need to worry about it. That's assuming the number even needs to be said at all. There are plenty of ways around it.