Mobile service is likely your largest single cost of smartphone ownership. Shopping around to shave money off your bill can really add up over time.

People rarely switch cellphone providers, though. Doing so is confusing and time-consuming. I’ll tell you about a shopping comparison site that can make it a little less painful.

For the past few years, the big three mobile carriers in the United States — AT&T, T-Mobile and Verizon — have been trying to grab customers by dangling generous promotions on new phones if you pay for a higher-cost unlimited-data plan.

If you’re among the majority of Americans who buy new phones on a monthly plan from the big three companies, you’re typically paying up to hundreds of dollars less than the sticker price for a new iPhone, according to BayStreet Research.

This chart shows that big three wireless customers are now paying an average of one-third off the sticker price for a top-tier iPhone, after counting the trade-in value of an older device, discounts and other goodies.

You’re generally paying less than the list price for other higher-end phone models, too, though promotions are typically heaviest for iPhones.

Are the phone company device discounts worth it? It depends.

Adeeva Fritz with the phone plan comparison site Navi gave the example of two different unlimited data plans from AT&T.

The AT&T Unlimited Starter SL plan costs $65.99 a month for one phone line if you set automatic payments and skip a paper bill. Fritz said that’s the lowest-cost AT&T service plan that qualifies for as much as $1,000 off a new iPhone 15 model if you have a relatively recent smartphone to trade in. You get a monthly credit on your bill over three years for the value of the discount.

Another AT&T unlimited data plan, Value Plus VL, won’t get you a free iPhone but it costs $50.99 for one phone line. Over the same three years, your phone service is $1,836, or $540 less than the more expensive AT&T plan

If you want a new iPhone, have a relatively recent phone to trade in and don’t mind being committed to your phone company for three years, the higher-priced plan might be worth it.

If you tend to hold onto your smartphones for more than three years, don’t have a high-value phone to trade in and don’t want to be tethered to your phone company, you might be better off with a cheaper cellphone plan that has smaller or no device discounts.

Fritz’s example helps show two things: First, phone service is probably the largest cost of owning a smartphone. Even a top-tier $1,200 iPhone may be far less than the thousands of dollars you’re paying for phone service over a few years.

And two, it’s mind-numbing to figure out the phone plan that will make you happiest and save money. The number of people in your household, your data use and your phone preferences can dramatically alter your math.

That’s where Navi can help. Telecom analyst Craig Moffett of MoffettNathanson Research recommended the comparison site.

Enter details of your current phone plan, and Navi will walk you through the pros, cons and costs of alternatives, including other options from your current provider. If you want a new phone, Navi will assess the price you might pay at multiple phone companies.

You may have to do more research on your own, including for smaller phone providers like Consumer Cellular. It and many other smaller mobile providers, including the cable and internet providers Xfinity and Spectrum, use the same mobile networks as the big three. Your mobile internet speeds might slow down if networks are congested, though.

Check out more advice in the mobile plan buying guide from Consumer Reports. If you’re paying off your smartphone in installments on your phone bill, it might not be worth switching phone plans or providers until the device is paid off.

The hassle of mobile plan sleuthing could pay off. A survey of Consumer Reports members found that the majority of them who changed phone service in the prior year saved money — some more than $40 a month — and generally got better service.