With Google’s announcement Wednesday of its groundbreaking wireless service, details of the plan have been revealed. With the operation of Project Fi, Google will become a mobile virtual network operator (MVNO), creating pause for major operators like Verizon and AT&T.

The service is designed to switch transparently between the existing mobile networks of T-Mobile and Sprint, as well as Wi-Fi. A significant feature will allow users to connect their phone number to any Wi-Fi-enabled device, allowing talk and text no matter what sort of device is used.

Twenty dollars per month will provide access to talk, Wi-Fi tethering, and text in over 120 countries.  Also, in a major departure from the norm, users will pay only for the mobile data they use. This is in contrast to the standard convention of charging a lump sum each month for data, whether or not it is used.

Here is an  example, using the expected rate of $10 per gigabyte (GB): If a customer purchases 2GB of data for $20 but only uses 1.6GB, Google will refund $4.

Initial participation in the Early Access Program of Project Fi will be limited to those with invitations from Google and only by those in possession of a Nexus 6 phablet (a device in between a mobile phone and a tablet). The device was developed by Google and Motorola specifically for Project Fi.

Google does intend to become a significant player in the carrier market, instead creating new practices it would like to see the rest of the wireless industry adopt. A spokesman for the company has said Google has an expectation that carriers will embrace some of the new practices protocols, like phone calls automatically moving to Wi-Fi when they’re dropped by a mobile network.