By Ryan Knutson 

It is almost impossible to imagine life without a cellphone. But imagining life without a cellphone bill is getting a lot easier.

For 30 days beginning in January, I disconnected my iPhone from AT&T Inc. and lived entirely off the free Wi-Fi connections that are available nearly everywhere. It wasn't exactly seamless. An effort to dump extra tickets to the NCAA College Football Championship game in Dallas was more stressful than it would have been with an always-connected phone. On occasion I had to lean on friends who were still on the network. But with a few changes in habits, it was definitely doable.

So doable, in fact, that it would be surprising if wireless carriers aren't already starting to worry that their industry could one day face a version of the cable-television business's cord-cutter problem.

If millions of TV watchers are making do with a money-saving patchwork of online shows, over-the-air broadcasts and services like Netflix Inc. how long will it be before wireless subscribers decide they can get by making calls with Google Voice, sending texts via WhatsApp and asking for the Wi-Fi password every time they enter a bar?

The question isn't theoretical. Thousands of people in the U.S. already are. In addition, the majority of mobile data traffic flows over Wi-Fi, and more access points are being built daily. There are commercial efforts to build cellphone businesses on Wi-Fi, too, from companies such as startup FreedomPop and pay-TV heavyweight Cablevision Systems Corp.

In Europe, low-cost operator Iliad SA has used Wi-Fi in part to help increase its share of the French mobile market. Seoul offers free Wi-Fi across its public transit network, part of the proliferation of hotspots across Asia.

"The percentage of time that a phone is in Wi-Fi coverage is very high," said Rick Osterloh, president of Motorola Mobility, the cellphone pioneer that was sold to Google Inc. and then Chinese computer company Lenovo Group Ltd. "This is the beginning of a trend."

Of course, carriers aren't going away anytime soon. Verizon Communications Inc. said wireless revenue rose 8% to $88 billion last year. AT&T's climbed 6% to $74 billion. Together, they added a total of 8.8 million mainstream subscribers, about three million more than a year earlier.

There are real hurdles to relying only on Wi-Fi. Not every building has it. Most outdoor areas certainly don't, so talking on the phone while traveling in a car or walking down the street can be nearly impossible. Logging into a Wi-Fi network generally requires a password, and there are millions of them. If the power goes out, the Wi-Fi goes out along with it because most hotspots don't have backup battery power.

But at $0 a month it is hard to beat the price.

The threat that inferior services pose to established companies was explained by Harvard Business School professor Clayton Christensen in the 1997 book "The Innovator's Dilemma." Incumbents gear their services toward getting the most money out of their best customers. That leaves an opening for cheaper services to find a market around the edges. Eventually, the inferior product improves enough that its audience is in the majority, and the established players are marginalized.

Mr. Christensen said in an interview that he has been waiting for that scenario to take root with Wi-Fi and the wireless business. "This is something that we've watched for a long time," he said, "and frankly it has moved more slowly than we'd expected."

My experience playing the role of the marginal customer was relatively painless. Most buildings in New York and on a trip to Dallas had hotspots. Calls can be forwarded free to Google Voice or to Skype if you sign up for a plan. Internet apps like Apple Inc.'s FaceTime and iMessage, Google Hangouts, and Facebook Messenger make it easy to stay in touch with friends and family. Even the little blue dot on Google Maps will track your location without cellular service. It works in part by mapping Wi-Fi hotspots, in fact.

There is a lot of technical help out there for people trying to cut the wireless link. A $5 app, Wi-Fi Map Pro, shows the locations of nearby hotspots even when your phone is off line, and it provides passwords to some private networks. I used it, for example, when a friend was 45 minutes late for dinner at a ramen shop in Manhattan that didn't have Wi-Fi.

Companies such as iPass Inc. and Devicescape have stitched millions of hotspots together globally into a networks with a single sign-on. Major U.S. cable companies have built millions of hotspots around the country and their home Internet customers get free access. And several startups are trying to ease phone-call handoffs and solve the password problem to enable more widespread use.

The experiment did require a few changes in habit. I had to plan my route before leaving home since accessing directions on the spot while I was out wasn't guaranteed. I also called restaurants and shopping centers in advance to see if Wi-Fi was available and asked for passwords when I arrived. And I had to set and keep a time for meeting friends.

A few times I had to give colleagues a friend's phone number so I could be reached. Once, I had to skip my usual basketball game because the gym didn't have Wi-Fi, and I needed to remain reachable via email on deadline.

Adjustments such as those are inevitable.

The high price of mobile service led Charissa Struble of Arkansas City, Kan., to give up Verizon Wireless and make the switch to Wi-Fi only. "I just got really tired of paying an exorbitant phone bill," she said.

In September, Ms. Struble bought a $99 phone from a company called Scratch Wireless that enables Wi-Fi calls and texts from the minute it is turned on. When Wi-Fi isn't around, unlimited text messages are provided via a deal Scratch struck with Sprint Corp.

"Almost everybody offers Wi-Fi now, so I haven't really had an issue," said 37-year-old Ms. Struble. Scratch also sells cellular day passes for $1.99. She has bought them a few times to gain access during long car rides.

Big carriers are starting to address the issue, emphasizing that only their networks can keep subscribers connected at all times, wherever they are. Peter Ewens, executive vice president of corporate strategy at T-Mobile US Inc., told Wall Street analysts in late February that Wi-Fi can supplement cellular service but can't replace it.

"We don't see Wi-Fi per se as a threat," Mr. Ewens said. "We see Wi-Fi as a natural complement."

AT&T and Verizon say consumers have an insatiable demand for connectivity and will continue to pay for the ability to stay connected wherever they go.

Wall Street is also starting to pick up on the trend. Craig Moffett, senior analyst at MoffettNathanson, says that one day Wi-Fi will be the primary network, and cellular will serve as the fallback when Wi-Fi isn't around. "That shift may sound subtle, but it will usher in a period of profound disruption," he said in a research note in October.

Jonathan Chaplin, a telecom analyst at New Street Research, estimated in a report last year that Wi-Fi-first mobile services offered by cable companies in the U.S. could devour more than 15%--or $68 billion--of the equity value of U.S. wireless carriers within the first five years of launch.

There is no question people around the world have become accustomed to always being on. But there is a difference between habit and necessity. Some researchers, such as Harvard professor Leslie Perlow, say that being connected constantly can hinder productivity. Other experts, such as David Greenfield, founder of the Center for Internet and Technology Addiction, say it can lead to smartphone addiction. Some people are beginning to push back socially by doing things like banning cellphones during dinner parties.

After getting used to being unconnected for brief periods, I became less attached to the device itself and found myself checking my phone less often, even when I had service.

Scott McClain, who was paying Verizon for his home Internet and his cellphone service, switched himself and his two children to Wi-Fi last year. He said it seemed like a "no brainer."

"The thing that just drove me nuts was paying for the Internet twice," said Mr. McClain, a 49-year-old electronics technician who lives outside Pittsburgh. "No matter where you go, there's always Wi-Fi."

His 13-year-old daughter Hailey says she doesn't plan to ever pay Verizon or AT&T for cellphone service.

Four years ago, David Morken co-founded Republic Wireless, which offers mobile service primarily over existing Wi-Fi. In places where Wi-Fi is unavailable, the service uses Sprint's network. Mr. Morken says he set out to create a cheaper alternative to carriers once he realized he couldn't afford to pay the cellphone bills for his six children.

About 38% of his customers choose a $10-a-month plan that gives them unlimited calls and texts on the Sprint network but only Wi-Fi for data traffic. The company now has several hundred thousand subscribers and is growing by about 14% a month.

"We're carving a lane," Mr. Morken said.

Write to Ryan Knutson at ryan.knutson@wsj.com

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