Google is offering a wireless phone service designed to pressure major carriers such as AT&T and Verizon Wireless into lowering their prices.

The service, called “Project Fi,” will cost $20 per month and only charge customers for the amount of cellular data that they use each month instead of a flat rate. Each gigabyte of data will cost $10 per month. That means a customer could sign up for a plan offering three gigabytes of data and get $20 back if only one gigabyte was used in a month.

Most wireless phone carriers allow their customers to roll over unused data into another month of service without refunding any money.

Google’s service initially will be available only on the Nexus 6, a Motorola phone made with Google’s help.

(AP)