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Majority of Americans now use ad blockers
(www.theregister.com)
This is a most excellent place for technology news and articles.
If the phone costs $500, they simply increase your monthly bill by $500 / 24 months = $20 a month.
It's a bit more complicated than this, and they'll likely have some interest built in as well, but functionally, it's no different than being given a loan to buy the phone and then paying the loan off over the two years. That's why carriers often require a credit check before doing this.