YOUR household Wi-Fi router has a hidden power: It is also a cellphone service provider. In fact, most mobile data traffic travels through smaller networks like Wi-Fi routers, not those hulking cell towers outside.

The internet companies Google and Bandwidth.com have turned that little-known fact into a business opportunity. Their mobile phone services rely on cell networks that they lease from traditional carriers, but if a Wi-Fi network has a better connection, they shift phone calls and data over to Wi-Fi instead.

More often, your Wi-Fi is going to have a stronger data connection, so in tech industry parlance, these phone services are sometimes called “Wi-Fi first.”

Less work on cell towers leads to lower costs for consumers, so Google and Bandwidth offer service prices that are less than half of what a traditional carrier would charge. Bandwidth’s phone service, called Republic Wireless, offers a range of plans, including one for $25 a month for unlimited minutes and messages and 1 gigabyte of cellular data. Google’s service, Project Fi, costs at least $30 a month for the same package, and you are reimbursed for the cellular data you don’t use.