The Wall Street Journal says:

The service, designed to switch among Wi-Fi and cellular networks, will initially be available only on the latest Nexus 6 smartphone designed by Google and made by Motorola Mobility, a former Google unit now owned by China's Lenovo Group Ltd. , two people familiar with the matter said. One of the people said the service won't work with older Nexus devices, such as LG Electronics Inc. 's Nexus 5.

When Google's Sundar Pichai spoke about the project at Mobile World Congress earlier this week, he stressed that any potential MVNO offering from the company would be limited in scope, serving as more of a proof-of-concept for integrating Wi-Fi and cellular service. However, if The Wall Street Journal's sources are correct, restricting access to the network to the Nexus 6 could mean that the project will be much more limited than expected.

However, as with all reports of this nature, this news should be taken with the requisite grain of salt. The service was only briefly touched on at Mobile World Congress, and any plans could change a hundred times between now and its launch.

Source: The Wall Street Journal