The service will switch between Wi-Fi, Sprint and T-Mobile. A pretty tough piece of networking when you consider that Sprint is a CDMA network and T-Mobile is a GSM network. Because of all the network hopping, Google needs to tweak the hardware and software of devices that use its network. This is why the Nexus 6 will be the device of choice, Google can’t get in to tweak the settings of phones from other manufacturers.
Google’s carrier ambitions are more of an experiment into what can be accomplished than a full-on assault on carriers. So don’t expect to start seeing Google running ads with unlimited data, text and voice offers anytime soon.