Smartphone manufacturers and mobile carriers are natural partners, with a shared interest in selling you stuff. The mobile carriers advertise big discounts and even claim they'll give you a new phone "on us." But are those deals legit or too good to be true? That can be impossible to figure out without an advanced accounting degree.

As I wrote earlier this month, there are lots of gotchas hidden in these deals.

I regret to inform you that you did not win the lottery, and your mobile carrier is not doing this out of the goodness of its giant corporate heart. This is a business deal, and you can bet that somewhere on the top floor of that carrier's headquarters there's a spreadsheet that shows exactly how much profit they stand to make off you. ...

In my experience, carriers go out of their way to make these deals complicated, and you need to look carefully to see if there are any hidden gotchas.  

That post includes seven questions to ask up front to make sure you don't get tripped up by one of those sneaky requirements.

My real-world example 

My iPhone 12 Pro Max will be four years old later this year. It's still got excellent battery life, but I would love to take advantage of some of the camera improvements in Apple's latest model, theiPhone 15 Pro Max . With 512 GB of storage, that phone sells for$1399.

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I could just go to Apple's website and buy the latest model for cash, trading in my old phone for a relative pittance,$370. The total price after trade-in and before sales tax would be$1,029. Apple will happily let me pay off that phone over 24 months, at 0% interest, for a payment of$42.87 per month.

But Apple's website also offers an alternative, in the form of deals from its partners at the three major US carriers.

Apple says Verizon will give me$830 for my old phone. At Verizon's website, I'm offered a$1000 trade-in credit. Meanwhile, my carrier, T-Mobile, is stingier, offering only$800. And those cheapskates at AT&T are obviously out of the running with their measly$700 trade-in allowance. But all of those offers have lower monthly payments than Apple is asking, so they must be better deals, right?

Well, not exactly.

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That T-Mobile offer turns out to be the clear winning deal for me, with Apple a close second. If I take my business to AT&T or Verizon, by contrast, I will wind up paying hundreds or even thousands of dollars more than if I had just bought the phone with Apple, at 0% interest.

Read the 'deal details'

The only way I know that is because I clicked those little links that say "See deal details," and then I did the math. Or, to be more precise, I created a spreadsheet that helped me figure out the true cost of a smartphone deal.

You can see all the details for my deal here: Smartphone cost comparison

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Over the course of three years at Verizon, I would pay$2,919 for that$1399 phone. At AT&T, I'd pay$1,851.

Yikes! What happened to all those sizzling deals? Blame the fine print -- specifically, the part that requires you to "connect on an eligible rate plan" to qualify for the full discount. The monthly payments quoted in those tempting deals are only for the device. The bill you actually pay each month also includes your wireless service plan. And that's where things can get very expensive.

At T-Mobile, my monthly bill for two lines of unlimited service is$90 a month, with no additional taxes or fees. That's on a legacyMagenta Max 55+ plan , which the company no longer offers. If I upgrade to the newerGo5G Plus 55 plan for an extra$10 a month ($5 per line), I qualify for that$800 discount, or$33.33 per month. Over the course of two years, even with the pricier plan, those discounts save me$190 over what I would pay if I bought the phone from Apple directly and stuck with my old plan.

At AT&T and Verizon, the math is not so kind. To get those promised discounts, I need to sign up for the most expensive unlimited plans the two companies offer. At Verizon, that's$160 a month (after discounts for autopay and paperless statements) for two lines on theUnlimited Ultimate plan . Over at AT&T, I would pay$122, after discounts, for two lines on theUnlimited Premium PL plan . And the monthly bill at either of those carriers would be about$13 higher after adding taxes and fees.

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If I'm willing to switch carriers to get those discounts, I'm going to pay an extra$45 to$83every month for three yearsjust for the wireless service plan. And I need to keep my line active for three full years if I want to receive all the bill credits that are included in the deal. That's why the supposedly discounted device actually winds up costing so much.

What if I'm ready to change carriers, even if it means paying extra for a new plan? In that case, I might be better off buying that phonedirectly from Apple online and then taking advantage of the Bring Your Own Device (BYOD) discounts from my new carrier. At Verizon, that's$30 a month for two lines, for three years; at AT&T the BYOD discount would be$20 a month. In either case, I would pay less for the phone in the long run and I wouldn't be locked to my carrier for 36 months to collect those bill credits.

's Smartphone Deal Calculator

You can do these same calculations with deals from other smartphone makers, including Google and Samsung.

Your mileage may, of course, vary. If you sign up for four lines on an unlimited plan at Verizon or AT&T, for example, you'll get a much better deal than if you have two lines like me. Every situation and every smartphone offer is different; that's why I've shared a copy of this spreadsheet, so you can make your own calculations using your own numbers.

The link is here:'s Smartphone Deal Calculator

Note:It's a read-only document, so you'll need to click File > Make a copy and save it in your own Google Drive to enter your data.

You can compare up to four scenarios. Enter a value in each column on the rows that are shaded in yellow, and be sure to include the cost of your monthly plan at the top.

For trade-ins, some carriers provide some trade-in value as an instant credit that comes off the cost of the new phone, with the remainder set as bill credits. Make sure to enter these values as negative numbers (with a minus sign in front) so that they're calculated correctly.

After years of studying these offers, I've found that truly good deals are hard to come by, but they are out there, and it pays to keep looking. Happy hunting!

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