Google's mobile phone carrier service, Project Fi, has gone live and the app is now available in the Play store.

The service, which piggybacks on the Sprint and T-Mobile networks and local Wi-Fi, is available to anyone with a Nexus 6 phone. The Project Fi app has appeared on the Google Play store, but it won't work without an invite.

Anyone with an Android phone can download and install the app, but as Google said from the start, it will only work on a Nexus 6. The app won't even open on the Samsung Galaxy S5. The Android Application Package (APK) is also on APK Mirror for anyone interested in taking a closer look at the app.

The complexity involved in hopping between Sprint and T-Mobile and Wi-Fi is the reason Google can only run the service on its native Nexus phone. Google says this is because the Nexus is the only smartphone that supports its network of networks.

Phones on the network will try to connect to a nearby Wi-Fi signal first and only default to LTE from Sprint or T-Mobile if no Wi-Fi is available. Google has promised that there will be more than a million free Wi-Fi hotspots across the United States.

The service costs $20 per month for "Fi Basics" (including unlimited talk and text, Wi-Fi tethering and more), and Google is charging a flat $10 per GB of data on top of that.

The app lets you activate service, tweak your account settings, check how much data you're using and see your monthly bill, among other things.

For now, unless you have an invite, the app is useless. So far, only a few Googlers have invites to the service, but the fact that the app has appeared on the Play store could mean that Google is ready to roll out its first wave of invites soon.