Although it will offer mobile services, Google hasn't built its own network for this purpose. Instead, it will act as an MVNO (mobile virtual network operator), using the infrastructure of both Sprint and T-Mobile.
This makes Google a pretty unique MVNO - and means devices connected to Project Fi need to support the network bands for both T-Mobile and Sprint. This isn't something every phone out there can do, so for now Google is only allowing Motorola Nexus 6 units to use Project Fi.
If you already own a Nexus 6, Google will send you a SIM card. If you don't have the phone, then you can buy one through Project Fi on a 24-month installment plan (with payments of $27.04 for the 32GB model, and $29.12 for the one with 64GB of storage). There's also the option to pay in full - $649 or $699, depending on which storage variant you choose.
Project Fi lets you make calls and send and receive texts via Wi-Fi when you're connected to such a network, so even if you encounter no signal areas you can still use your phone as you normally would, provided that you have a usable Wi-Fi connection. Calls are said to transition seamlessly between Wi-Fi and the mobile networks.
Google's service has just one plan, and no long-term contract requirement. You pay $20 per month for "The Fi Basics" and then you add a data allowance on top of that. The Fi Basics consist of unlimited domestic talk and text, as well as unlimited international texts. Data costs $10 per GB, and you can choose bundles in increments of 1GB - from 1GB all the way up to 10GB. Overages are charged at the same $10 per GB rate, so if you go over by 300MB, for example, you'll pay an extra $3.
Wi-Fi tethering is built-in. The data costs are exactly the same even in more than 120 countries across the world while you're roaming, but in this case your speed will be limited to 256kbps. Obviously in the States you're on Sprint's or T-Mobile's 4G networks, so you should get much more.
If you don't use all the data in your allowance in a month, you'll be credited the difference in your Project Fi account, and you can use that towards your next bill. If you pay $20 for 2GB but only use 1.5GB, you'll get $5 back. You can change the data package from month to month, and all account administration is done via a dedicated mobile app which you install on your Nexus 6.
The huge caveat right now is that you need an invite to get in on the action. Yes, Google is once again pulling its Gmail trick, so if you want to use Project Fi you have to request an invite from its official website and hope for the best.
MVNO is not exactly a new thing. There are several in the UK And states already.
WTF??? is MVNO, you can use one sim for both networks. this is not new, this time Google copied the business model from UK (i guess, they are the first), where MVNOs like Virgin Media are using the EE's network (t-mobile and orange network together...
i not not undertande it, but the google is so good