The move toward equipment installment plans (EIPs) could generate a total net outstanding balance of consumer loans of $55 billion across the four major U.S. carriers, Moody's analysts wrote this week. And while carriers who adopted EIPs early "have already addressed the majority of their shortfall," the loans don't incur interest and could actually end up costing operating money.

Verizon, which was the last major U.S. carrier to offer EIPs, is likely to experience the largest working capital deficit increases due to the loans. The nation's largest operator "could see its accounts receivable balance from EIPs double to about $25 billion, or about 20 percent of its current total funded debt balance," Moody's wrote.

T-Mobile was the first U.S. carrier to dump two-year contracts and subsidized handsets in favor of EIPs, and the strategy was so successful every other tier-one operator has followed suit. But as EIPs continue to gain favor among consumers, operators must be vigilant to ensure they don't get over-extended on consumer credit in an effort to grow their subscriber bases.

"EIPs increase financial risk because the wireless carrier is essentially underwriting consumer-finance loans," the analysts wrote in the in-depth sector report. "Carriers must carefully assess the credit quality of subscribers to determine the amount to extend, while balancing the need for subscriber growth. We see risks the companies could relax lending standards to attract or retain subscribers and boost near-term results, especially in a competitive, near-saturated market. The underwriting standards at issue are likely to be proprietary methods that are opaque to the public markets."

Indeed, Wells Fargo Securities said in March that bad debt from consumer credit standards had "caused a bit of disruption," taking a toll on the carriers' bottom line. Newly implemented policies appeared to be addressing the problem, Wells Fargo said, but T-Mobile conceded that credit-policy problems remained.

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