Google announced on Monday that it would open its Project Fi wireless phone service to interested potential subscribers for just one day.

Project Fi is Google's wireless service that uses both WiFi and cellular connections to offer affordable wireless plans for customers. Google announced it would be offering instant "invites" to customers wanting to subscribe to the service, CNet reports.

Before the open invite system, Project Fi was available only to customers who specifically requested more information on the service. The service was also only available for one phone at first, the Google Nexus 6.

When Google announced two new Nexus phones, the Nexus 5X and the Nexus 6P, the company decided to make them compatible with Project Fi.

Project Fi is very affordable, making it an attractive option for wireless customer hoping to save money. Unlimited calling and texting is just $20, and data is $10 per gigabyte. Unused data will be refunded as bill credits each month.

With the service, Google is offering an alternative to traditional calling plans. WiFi hotspots provide the primary service for Project Fi. Only when WiFi networks are unavailable or unreliable will Project Fi devices automatically switch to a cellular connection. Google is using Sprint and T-Mobile's networks for the cellular side of the service.

As subscribers move, technology built for Project Fi detects and uses the best network. Even when the device switches to cellular service, it will choose the strongest carrier network available, either from Sprint or T-Mobile.

Project Fi is also available for international calling. Customers will be able to call 120 different countries for 20 cents per minute and send unlimited international text messages. Additionally, customers will be able to use their same data plans while traveling at no additional charge.

For customers who typically do not use very much data, a Project Fi plan could be as low as $30 per month. Customers interested in Project Fi can find out more on the Project Fi site.