Here's something that may surprise you:

Switching to Comcast can save you money.

The cable and broadband giant, often criticized for its rising bills and fees, has one of the best deals for cellphone service on the market — if you don't use a lot of data.

There are some caveats. You have to be an existing Comcast internet customer to subscribe. And heavy use on multiple lines erases Comcast's advantage.

Xfinity Mobile, launched in May 2017, is a relatively new entrant to the cell phone market and is avidly trying to build up the business as it incurs substantial losses. It offers two options: a monthly $45-per-line unlimited talk, text and data plan, and a pay-as-you-go plan for $12 per gigabyte. Comcast also allows customers to mix and match plans on one account, meaning parents can choose pay-as-you-go plans while signing up their kids for unlimited data.

Comcast's unlimited plan is the cheapest single-line offer compared to the major wireless providers. Verizon, for example, charges $75 for a single-line unlimited plan. AT&T and T-Mobile both bill $70, while Sprint's costs $60, according to offers on their websites.

Xfinity Mobile's unlimited plan remains a bargain until several phones are added. Sprint sells three unlimited lines for $120, compared with $135 through Xfinity. At four unlimited lines, AT&T, Verizon and T-Mobile all charge $160, while Xfinity will bill $180.

But Comcast officials say many consumers don't need that much data. Randall Hounsell, Comcast's senior vice president of product and marketing for Xfinity Mobile, said roughly 60 percent of smartphones use less than five gigabytes of data, while a third use less than two gigabytes.

"In a family, there's usually a couple devices in there that don't need unlimited," Hounsell said. Giving consumers the flexibility to mix unlimited with per-gigabyte plans is "our smart way of allowing families to save," he said.

Verizon does not allow account holders to have some lines on unlimited data while others separately share a data plan. An AT&T spokesperson said the company does give customers that option, but acknowledged that such an offer "isn't online anywhere." T-Mobile offers only an unlimited plan. Sprint's website doesn't mention a mix-and-match option and did not respond to a request for comment.

Comcast customers can switch lines from a per-gigabyte plan to unlimited data at any point during the month if they find themselves using more data than expected. An Xfinity Mobile app allows users to track their data usage and easily make changes to their account.

Xfinity's $12 per gigabyte plan is also cheaper than the competition in several instances. For example, a Comcast account using three gigabytes of data per month would cost $36, less expensive than the $50 that AT&T and Verizon would charge. Again, T-Mobile does not have a shared data plan, so at a minimum you'd have to pay $70 for unlimited data. Sprint sells two gigabytes for $40, but you'd need to buy the $60 unlimited plan to reach three gigabytes.

There are cheaper data plans out there in some specific scenarios. For example, AT&T sells five gigabytes for $50 while Xfinity bills $60, and Verizon charges $70 for eight gigabytes compared to $96 through Xfinity. But Comcast doesn't charge access fees for up to five phones, while both AT&T and Verizon impose $20 per phone access fees. When factoring in those fees, Xfinity Mobile is still a better deal in those cases..

There are no contracts with Xfinity Mobile and no penalties for canceling, though customers who pay for their phones in monthly installments would have to clear their remaining balances upon quitting, a Comcast spokesperson said.

Xfinity Mobile runs over Verizon's wireless network under a 2011 deal that lets Comcast resell Verizon's wireless spectrum. Comcast is also utilizing its 18 million WiFi hotspots, so consumers don't always have to burn through their cellular data.

Comcast's move into the smartphone business is expected to cost the company $1.2 billion during its first 18 months. Comcast added 196,000 smartphone subscribers during the first quarter of 2018 and lost $189 million on the business. Comcast executives have said Xfinity Mobile will be profitable if the business gets larger.

As of April, Xfinity Mobile has 577,000 subscribers. The service is available only to Comcast's 26 million internet customers.

Walter Piecyk, a BTIG analyst who tracks the telecommunications industry, said Comcast hasn't dented the wireless market yet and would need to add about 325,000 subscribers per quarter for the industry to take notice.

"It's interesting and certainly a compelling service for Comcast customers, but they haven't taken enough customers from existing wireless operators to merit a competitive response to their actions," he said.

Piecyk described Comcast's move into the wireless market as both an offensive and defensive strategy. He said it helps the company hold onto internet customers and prepare for how 5G mobile networks will enable wireless competitors to encroach on the home internet business, while also generating free cash flow once the company builds up its customer base.

"Comcast would not have to achieve much market share in order to realize significant free cash flow from a successful wireless business," he said.