T-Mobile US is giving customers free devices that boost network coverage in and around their homes, with one caveat: the device lets anyone in the immediate area use the customer's Internet bandwidth.

T-Mobile said its 4G LTE CellSpot piggybacks on your Internet connection to "provid[e] an average 3,000 sq. feet of glorious full-bar T-Mobile LTE coverage throughout your home," even if there is no T-Mobile cellular signal in the area.

"[I]t’s the world’s first device of its kind that is 3G, 4G and VoLTE-capable, and any T-Mobile customer can connect to it automatically just like any other cell tower—no passwords or log-ins required," T-Mobile said in today's announcement.

We asked T-Mobile over e-mail and Twitter whether customers can configure the CellSpot to require a password or use a whitelisting scheme to limit access. We got an answer via Twitter, with the company saying, "The 4G LTE CellSpot doesn’t provide restricted access. Other T-Mobile customers within the device coverage range can also access the 4G LTE CellSpot coverage." (A T-Mobile spokesperson later confirmed via e-mail that there is no way to limit access to the device.)

The CellSpot is a new version of a device that was released last year. The previous one acted more like a regular Wi-Fi router, letting users specify a password. T-Mobile has distributed more than 1 million of them to customers.

The new 4G LTE CellSpot is free to Simple Choice PostPaid customers (with a $25 refundable deposit). To use it, customers should have Internet speed of at least 2Mbps downstream and 512kbps upstream. Customers set it up by connecting an Ethernet cable from the CellSpot to "any available Ethernet port on your router or router/modem combo unit."

T-Mobile said the device is also ideal for businesses and allows up to 16 simultaneous phone calls. If your home Internet is fast enough, T-Mobile says the CellSpot can support speeds of 60Mbps downstream and 20Mbps upstream.

Even though it's using your home broadband connection, any data you use while connected to the CellSpot counts against your T-Mobile plan's data limits. Customers must use Wi-Fi to avoid using up cellular data allotments—so you're better off using Wi-Fi to begin with, especially if your phone supports Wi-Fi calling. The CellSpot could also use up any data allotment you have for home Internet, if you're on a capped plan.

T-Mobile's network has traditionally struggled indoors. While the company has been boosting cellular coverage, the CellSpot and T-Mobile's embrace of Wi-Fi calling helps fill in some of the remaining gaps.

Since the new device's range is limited to 3,000 square feet, customers may not be sharing their Internet connection with too many people, especially if they live in a house in a sparsely populated area. In an apartment complex, it could be more of a problem. If other people are using your Internet bandwidth, it could slow down your own Web access.

AT&T, Verizon, and Sprint let customers restrict access

T-Mobile isn't the only carrier to provide devices like the CellSpot. AT&T sells the similar MicroCell, but phones must go through an activation process in order to use it, so not just anyone can connect. AT&T customers can also manage an approved user list to allow specific people to connect.

Sprint has an Airave device that also uses a customer's home Internet service, but lets customers restrict access to approved phone numbers. Verizon Wireless sells a Samsung Network Extender and provides a way to manage access. The Verizon system doesn't entirely cut off other users, but lets you choose who gets priority access.

T-Mobile's strategy here is similar to one employed by Comcast. Since 2013, Comcast has made its customers' home modems act as public Wi-Fi hotspots by adding a second signal to each device. Other Comcast subscribers who are close enough to the device can log in to the Wi-Fi network by entering their Comcast usernames and passwords.

According to Comcast, home Internet customers don't lose any bandwidth because the public Wi-Fi portion is separate. The public hotspot feature is on by default, but Comcast lets customers disable it.