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.

But relatively few Americans have adopted these alternatives, which are called voice over Internet protocol services, or VOIP, for short. Some consumers avoid the move because VOIP services can’t connect to 911 emergency call centers in the traditional manner, and must use workarounds. Others worry that if their Internet service goes out, so does their phone service.

In addition, the stability of the VOIP providers isn’t certain. Vonage itself has been battered by legal problems and another VOIP service, SunRocket, shut down this week.

I’ve been testing a new type of VOIP option that will go on sale in September from a Silicon Valley start-up called Ooma, whose product goes by the same name. It differs radically from Vonage and other current VOIP providers, in two ways.

First, Ooma is a $399 piece of hardware that you pay for only once. There are no monthly bills. You just buy an Ooma Hub, a small device that looks like an answering machine. You plug it into your Internet connection and attach a phone, and you get free, unlimited domestic calls, local or long distance, as long as you keep your Ooma.

Second, with Ooma, you can easily keep your regular phone service as an integrated backup, for 911 calls, and in case the Internet service in your home goes out.

Ooma combines the VOIP and regular phone service. If you keep your standard phone service, Ooma uses your current phone number. And, if you dial 911, it always places that call over the traditional phone network. During an Internet outage, the device seamlessly switches to use the regular phone service, but you still pay no fees to Ooma.

If you do keep your standard service, you can reduce it to a very basic, low-cost plan, just for 911 and backup. International calls are routed through the Internet by Ooma and the company says they will cost roughly what Internet phone services like Skype charge for nonmember calls, which is well below traditional landline rates.

Ooma also delivers some added benefits. It gives you a virtual second line. If a call comes in when you are already on the line, the second call can be answered from another extension. It also has a built-in answering machine, and allows you to check your messages and call logs online.

I’ve been testing Ooma in my home for about a week and, except for a problem on one phone jack, I found it worked as promised. I tested it with both corded and cordless phones, and I also tested a companion $39 device, called an Ooma Scout, which must be plugged into the phone jacks in your house you want to use, beyond the jack to which the Hub is connected. Each scenario worked well.

When I plugged my cordless-phone base station into an Ooma box, all of the remote handsets continued to work normally. The only difference was the dial tone; Ooma gives you a unique musical dial tone to tell you it’s on duty.

Ooma works using the peer-to-peer Internet system popularized by file-sharing sites. Each Ooma box is part of Ooma’s network. The box in your home, for instance, might carry someone else’s phone call, though you can’t hear that call, and this doesn’t interfere with your own ability to make and receive calls whenever you want. In my tests, the Ooma didn’t seem to affect the speed of the Internet connection used by our computers.

To build its network, Ooma will be seeding the country with 1,500 boxes over the summer. These will be provided free of charge. But the only way to get one, if you aren’t on the initial list, is to know somebody who has one. Each recipient gets three tokens — redeemable for a free Ooma — to give to others.

Set-up is relatively straightforward and the manual is clear, assuming you have standard cable modem or DSL Internet service.

I did run into one problem. When I plugged my cordless-phone base station into an Ooma Scout, outgoing calls worked OK, but incoming calls wouldn’t work properly. This problem cleared up when I moved the base station to a different phone jack, but it suggests that, at least on some jacks, Ooma may fail.

The Ooma devices can be constantly updated over the network to fix problems and add capabilities, and the company is planning to add more features and options, some of which may cost money.

Of course, there is no guarantee that Ooma can handle a large number of customers as well as it did my test unit. But Ooma may be a good option for people who want to cut their phone bills, and either aren’t worried about 911 and Internet outages, or are willing to keep a basic, low-price standard phone service to cover those contingencies.

• 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.