All Things Digital

The poor landline home phone is getting less and less respect. Increasing numbers of people don’t even have traditional landline phone service anymore. These folks prefer to rely on their cellphones, which can be cheaper to use and carry a number that travels with a person instead of being locked to a house.

Many others keep their landline-phone service grudgingly, only because it is needed for things like fax machines. But even they often use their cellphones at home, because their friends and family members dial their cellphone number routinely, and their personal phone books are inside their cellphones.

But there is a big drawback to using a cellphone at home, especially in a large house: You have to schlep it around with you from room to room. By contrast, landline phone service can be used via either cordless or corded extension phones. Now, T-Mobile, one of the big U.S. cellphone companies, is rolling out a new system that it hopes will make cellphone service at home more convenient and even cheaper to use.

The service, being introduced this month in two test cities, Seattle and Dallas, allows you to use a cellphone account with any corded or cordless home phone, with multiple extensions, for just $10 a month. That very low price gets you unlimited domestic calls.

This new T-Mobile service, tentatively called Talk Forever Home Phone, is likely to be available nationally in a few months. It works via a special Wi-Fi wireless router that you must buy, with a two-year contract, for a one-time charge of $50. The router, which can either replace or supplement your existing wireless router, is essentially a stationary cellphone that marries an in-home Wi-Fi network to the T-Mobile cellphone network.

I have been testing the new system and found that it worked well, and it was extremely simple to set up and use. For my tests, I used a cordless phone supplied by T-Mobile, which included a base station and one extension handset. I was able to make and receive calls all over my home in exactly the same manner, and with exactly the same quality, as I do with my normal cordless landline service.

While T-Mobile is selling this cordless phone as a $60 option, it isn’t necessary for use with the new $10 service. The only new hardware that is required is the special Wi-Fi router.

However, there are some significant downsides to the new T-Mobile service that might make people think twice about dumping their landlines. For one thing, it doesn’t work with fax machines, home-security systems and other devices that rely on dial-up modems. Also, unlike landline phones, it doesn’t automatically transmit your home address to 911 emergency centers. You have to manually supply that address to T-Mobile during signup, and the company then sends it to your local emergency center.

Another downside: You must be a T-Mobile cellphone customer to buy and use this $10 monthly home service, and your T-Mobile plan must either be an individual plan costing at least $40 a month or a family plan costing at least $50 a month.

Finally, while you can transfer your current landline phone number to this new service, it cannot share your existing T-Mobile cellphone number. So people who are used to calling you on your cellphone will still do so, and you will still have to race for the cellphone or carry it around to receive those calls. You also can’t transfer your cellphone’s address book to the new home phone.

The special router is made by Linksys and looks very much like a typical Linksys router, except for the fact that it has two standard telephone jacks in the back and slots inside for T-Mobile SIM cards, the same kind that are inside a T-Mobile cellphone.

You can use the special router as a replacement for your current Wi-Fi router, but I just plugged it into an existing port on my old router, inserted the SIM card, and then plugged the cordless-phone base station into one of the phone jacks. It worked immediately, and didn’t affect or degrade my existing Internet service.

In addition to enabling the phone service around my house, the router was also usable by my computers for Internet connectivity, though it doesn’t support the new, fast “n” flavor of Wi-Fi.

This new system is not a so-called voice-over-Internet-protocol phone system, such as Vonage. It doesn’t carry your phone calls wholly over the Internet, but merely uses the Internet to get them to the T-Mobile cellphone network, which then carries the calls as if they had been made on a cellphone.

T-Mobile says the system will work fine even if you don’t have T-Mobile cellphone coverage at your house, because the call doesn’t rely on the cellphone network for its first leg and only is routed to the cell network once it reaches a T-Mobile switching center.

If you are a T-Mobile customer and can live with this system’s drawbacks, the $10 monthly fee may be hard to resist. But this new system is far from a perfect replacement for landline phones.

• Email me at mossberg@wsj.com. Find all my columns and videos online, free, at the new All Things Digital Web site, http://walt.allthingsd.com.