credit mule
n. A credit-worthy person who is recruited by a scammer to sign a cellular contract to get a free or discounted phone and then sell that phone to the recruiter.
Other Forms
Cash-desperate consumers are being lured into becoming unwitting "credit mules" via a cellphone scam, the Federal Trade Commission warns.
—Ellen Ambrose, “Don't Become a Credit 'Mule',” AARP, June 11, 2014
Your grifter word of the day is "credit muling" iPhones at the Apple Store.
—7rider, “Your grifter word…,” Twitter, April 29, 2014
In a new twist on identify theft fraud, a Michigan operation allegedly used "credit mules" posing as new or existing cell contract customers to scam thousands of free or subsidized phones from AT&T, Verizon Wireless, Best Buy, Radio Shack and Apple stores.
2012 (earliest)
These individuals are being recruited to buy the iPhones for $200, while sacrificing their credit to get the phones, in return for a cash payment," King said. …

A representative from Verizon told King they intend to move forward in the case against Blockmon and Staten, who was referred to as a "credit mule.
—MarieSam Sanchez, “Venice Man, Two Others Arrested for Alleged iPhone 'Gaming' Scam,” Venic-Mar Vista Patcj, May 29, 2012
The credit muling scam works like this: A scammer — called the "recruiter" — convinces a cash-strapped but credit-sound person — the credit mule — to visit a cellular provider and sign up for a contract that offers a free or discounted smartphone. The recruiter buys the phone from the credit mule and tells the mule to cancel the contract at the end of the month. The recruiter then unlocks the phone and sells it for a huge profit. Later, when the mule tries to cancel the contract but can't produce the phone, he or she is on the hook for the cost of the phone and any usage charges that have accrued.

This is sometimes called the "gaming" scam, presumably because the scammers are gaming a system that offers a free or low-price phone with a contract. Another variation is to recruit homeless people to obtain subsidized phones by signing new or updated contracts using identities that have been stolen by the recruiter.

Why a "mule"? Because a mule is a pack animal often used to carry things, and in the crime world those who smuggle contraband have long been known as drug mules, or just mules. The gaming scam, in a sense, "smuggles" a person's credit to obtain a free or cheap phone, so that person becomes a credit mule.

In his monumental (and, alas, unfinished) work The Random House Historical Dictionary of American Slang, J.E. Lighter includes a longish entry for the various slang uses of mule. Here's definition 4:
A person who smuggles or delivers contraband, esp. illicit drugs, usu. as a low-level member of a criminal organization.
Lighter's earliest citation comes from Emily "Janey Canuck" Murphy's 1922 book Black Candle, although when you look at the full citation, you see she's actually quoting an earlier source:
Bert Ford in The Boston American, writing of drug-intoxication in Boston says, "The 'mules' and 'joy shots' are among the most vicious elements in the plague."