Done properly, it could eliminate slamming
"this would make slamming (moving customers onto a new network without their knowledge) a great deal easier"
I disagree. Done properly, a central database could ELIMINATE slamming.
The key is that the central database must be controlled only by the end users, not to any of the companies involved in providing the service.
A customer wants a mobile number, they sign up at the central database and the number is issued. They assign a password, and they choose a service provider for that number.
Switching suppliers requires the customer to log back in on the central site and choose another supplier. There's no need for complicated "outgoing supplier must agree" procedures -- the phone owner has sole control.and the only third party that has to "agree" is the supplier taking over the service.