Technically, Wi-Fi calling depends on just the carrier and the smartphone. This means a Wi-Fi-calling-enabled smartphone can place and receive voice calls or send and receive text messages using any Internet-ready Wi-Fi network. However, T-Mobile says the CellSport comes with patent-pending technology that prioritizes voice calls over other Wi-Fi traffic. This means with the CellSpot, you can make qualitative voice calls even when the router is busy performing other tasks, such as media streaming or heavy file downloading.
Wi-Fi calling brings cell coverage to a whole new level. This is because, to connected smartphones, the Wi-Fi signal coverage is now also the cell coverage. And since it's much easier to bring an Internet connection to places where cell signals can't reach, such as the lower levels of a building or the basement of a home, the CellSpot allows not just wider but also deeper cell signal penetration. It's also significant that this coverage also extends to outside of the US. Using Wi-Fi calling, you can use your T-Mobile phone overseas, without roaming, and as long as you're communicating with somebody in the US there's no extra charge.
The CellSpot excelled in my testing, both as a home router and as a device supporting Wi-Fi calling.
As a router, it actually outdid the Asus RT-AC68U by a small margin. This was likely because the RT-AC68U was tested in 2013 with older firmware.
On the 5GHz frequency band, at close range the CellSpot registered a sustained Wi-Fi speed of 571Mbps. When I increased the range to 100 feet, it scored 340Mbps. These are among the top Wi-Fi speeds I've seen.
On the 2.4GHz frequency band, the CellSpot actually topped the charts with 250Mbps and 214Mbps for close range and long range, respectively. Like the RT-AC68U, the CellSpot had an effective range of about 200 feet. And it also passed my 48-hour stress test, in which it was set to move data between multiple Wi-Fi clients: during this time it didn't disconnect even once.
Note that I tested the router at CNET's offices, where there are walls and many Wi-Fi devices that are out of my control. Generally, walls shorten the reach of a Wi-Fi signal, and other Wi-Fi devices create interference. As with all Wi-Fi routers, your results may vary depending on where you live.
As a Wi-Fi calling supporting device, the CellSpot was also excellent. Within its effective range, I was able to place calls and hold many long conversations without being disconnected. The call quality was basically the same as when I used a regular cell signal, though distortion was a bit more frequent. I also noted that Wi-Fi calling worked well even when the router was set to do other heavy tasks, including file downloading, and HD content streaming to multiple clients.
Note that Wi-Fi calling doesn't allow uninterrupted transition to a regular cell signal. This means if you place a call using Wi-Fi calling and then move away from the router, when the Wi-Fi signal is cut off, the conversation will be disconnected even when there is cell signal. This also happens the other way around.
T-Mobile's Personal CellSpot is a win-win device.
Though you don't need it to use-- most any router will do -- there's little reason not to get one, especially if you need a router for your home. Under the T-Mobile branding, this is one of the best home Wi-Fi routers on the market with excellent performance and great features. And the fact that it's basically free for standard T-Mobile users (the $25 deposit is refundable when you return the router) doesn't hurt.
But when you're done using data, the CellSpot is great for making calls, too. Sure, Wi-Fi calling has its drawbacks -- it can cause a lot of feedback on a conference line, for example, and voice quality can sound a bit off -- but it remains a convenient and effective way to get cellular service when you're off the network. That makes it a great accessory for T-Mobile customers.