Since 90% of Americans already have some form of cell phone service, according to research from Pew, the major wireless providers can only grow by stealing customers from each other.

With the pool of new customers largely tapped out, AT&T (NYSE:T), Verizon (NYSE:VZ), Sprint (NYSE:S), and T-Mobile (NASDAQ:TMUS) need to steal subscribers from each other. That's a daunting task, but not an impossible one, as a 2013 research report from WDS (owned by Xerox) shows that just over a third (36%) were considering leaving their wireless provider at the time of the March 2013 study.

The report also showed that if a user's current carrier raised prices by 10% that 69% of customers who were previously unlikely to switch would now consider leaving. And if another company offered to lower their bill by 10%, the study found that "only 31% of customers who were previously unlikely to switch could guarantee that they wouldn't leave for this saving."

That means a large percentage of the wireless market is actually in play, and market leaders AT&T and Verizon are at least somewhat susceptible to efforts by upstarts T-Mobile and Sprint. Consumer Intelligence Research Partners analyzed 2,000 U.S. subjects who activated a new or used mobile phone in the 90 days preceding four quarterly surveys covering the period of October 2013 through September 2014. The research company found definitive reasons people moved from one company to the other.

Why do customers switch?

CIRP found that AT&T and Verizon drew customers based on the perceived quality of their networks, wrote  on the research company's Huffington Post blog. T-Mobile, he found, draws users based on price despite people seeing its network as inferior, and Sprint brings customers in based on features such as as friends and family pricing and unlimited data.

"Based on CIRP analyses, AT&T and Verizon lose customers because of the cost of service and to a lesser extent, the structure of their plans," Levin wrote. "Over half of consumers switched from those two carriers cited cost as the primary reason, compared to less than 40% for other carriers. Almost 40% of Sprint and T-Mobile departing customers identify network quality as the reason to switch."

What doesn't make them switch?

While price and network quality can cause people to switch providers, there's one thing that doesn't.

"Customers do not change carriers because of customer service," he wrote. 

That may explain why that particular area has been, if not ignored, at least neglected by the carriers. Why invest in something that people are willing to put up with if there are other areas -- network and pricing -- that could bring in new customers?

Who is this good news for?

Though the CIRP data shows that all four of the major carriers have positives that might bring in new subscribers, Levin told Fool via email that the research did show some emerging winners.
"The recent data suggests that T-Mobile and to a lesser extent Sprint have done a great job of attracting new customers," he wrote.
For those two, however, attracting new users is only the beginning of the battle.
"Both, however, have had their ups and downs in terms of retaining existing customers, so in some quarters they avoid losing too many, while in others they lose many, mostly to AT&T and Verizon," Levin wrote in the email. "In the most recent quarter, T-Mobile had huge gains in new customers, and avoided losing too many."
The challenge, and what will ultimately decide whether Sprint and T-Mobile can make any long-term gains, is whether the two smaller companies can get subscribers from the big two to give them a chance.
"AT&T and Verizon enjoy much higher loyalty, so they keep existing customers well, but don't gain new customers at the same rate as the T-Mobile and Sprint," he wrote.
The door is open
The CIRP survey suggests that customers want a low price on a quality network. Sprint and T-Mobile offer cheaper service than their bigger rivals, but are perceived by customers as having inferior networks. That's true, according to the most recent RootMetrics Network quality survey for the first half of 2014.

Source: RootMetrics 

Verizon and AT&T have sizable leads, but Sprint and T-Mobile have been gaining, while the big two are falling back to the pack, as the chart for the same survey for the second half of 2013 shows.

 Source: RootMetrics

This is potentially good news for consumers, as the more affordable Sprint and T-Mobile are catching up in network quality to their higher-priced competitors. If the third and fourth place carriers can eliminate the quality gap (as well as the perception of it) then they should see large-scale customer defections. Were that to happen, Verizon and AT&T would have to lower prices. 
Perhaps at that point -- when all four companies have similar networks and similar pricing -- customer service will begin to matter again.