How are cell phone providers able to sell iPhones much cheaper than the Apple Store itself?

I'm looking online for an iPhone 6s. The Apple Store is selling it for $299 while you can get it from Verizon Wireless for only $31.24. How is that possible? Is there usually a catch to deals like this?

No, there's no catch.

The Apple store is selling it for that price because they're not connected to any specific carrier.

Verizon and other carriers offer those low prices to entice you to sign up with them instead of a different carrier. Since all those low prices require a 2 year contract, they're eating some of the cost of the phone in order to guarantee your business for 2 years.

That is $31.24 per month for 24 months… Or $749.76. And you might want to look at the Verizon Wireless site again - that is for a 6s Plus.

The Apple Store price there's the old subsidised price listing vs what Verizon is showing which is the new pricing options. Then there's the fully unlocked one where you pay the full price upfront (which would be $749 in this 6s' case)

That is more likely than not $31.24 in monthly installments of 24 payments, making the total $700+ bucks when it is all said and done at the end of two years of payments.

That $299.00 is for what? 16GB version or a refurbished phone? You need to compare Apple to Apple and not Apples and Unknowns, when comparing pricing.

If you read the contract fine print, you'll usually see that you are paying full price via monthly payments that are higher than the normal phone bill would be without the new phone. (This is the case with your $31.24 price)

They have learned from the car dealers. Try to ask a new car dealership how much a car is going to cost and they will tell you how much per month it will cost. They try to never say the total price, for fear of scaring the customers away!

A quick way to check is to look at how much sales tax you will be required to pay. AT&T has specials where you pay $1 for the phone. You however have to pay sales tax on the full retail price of the phone, That means that you will actually be paying full price over the length of the contract, but they have to collect the tax at the initial sale time.