Re: So innovative
It's not cloning, it's registering one or more devices to be notified of incoming calls and to be able to handle those calls. Those devices may well be on another service providers network, and have their own contracts.
1G was an awfully insecure system, as everything was readily interceptable by any device in the network - which is why that worked. The same technique would not work from 2G onwards. However SIP was invented to support VoIP calls, where a central server keeps a record of subscribers to a service, and attempts to notify all registered subscribers of events related to that service. In that way, multiple different devices, on multiple networks, can register to receive calls to an IP phone number, and the IP phone service provider will attempt to notify all subscribed devices and establish IP bearers (not voice circuits) to those devices that respond.
Google and Apple like it cos it moves the operators one step closer to being a dumb pipe, this is another attempt to put the brakes on that on T-Mo's part.