The wireless service that Google all but confirmed this week will only work on the Nexus 6 smartphone, according to a new report. The Wall Street Journal, citing sources familiar with the matter, says Google's service could launch as soon as late March. But it won't work with any phone other than Google's current flagship device, which is manufactured by Motorola Mobility.

During an appearance at Mobile World Congress on Monday, Google's Sundar Pichai said Google is working with "existing partners" to create an MVNO, a type of wireless service built on top of infrastructure owned by other carriers. The Journal reported that Google's service, which will switch between cellular networks and Wi-Fi, will be built on top of T-Mobile and Sprint's US networks.

Why limit Google's wireless service to the Nexus 6? The Journal suggests that building a phone that works well on an MVNO requires seamless integration between hardware and software, and other manufacturers often modify Android. Another possible reason: Pichai indicated Monday that Google intends to keep the service small — "We don't intend to become a carrier at scale," he said. Limiting your network to a single phone would be one good way to do that.