Bill to your cell phone? Be cautious.

If you're inclined to try one of the new ways to pay and bill purchases to your cell phone, you better hope that nothing goes wrong with the transaction.

That's our advice after the four biggest cellular carriers flunked a Consumer Reports spot check designed to gauge how well customer service reps explain and help resolve problems involving your billing rights, which are dictated by each carrier's service contracts and are not covered by federal billing protections governing other forms of payment, such as credit and debit cards.

Bill-it-to-your-cell is part of a growing collection of new ways to use your wireless phone or tablet to pay for purchases of "digital goods," such as smart phone apps, downloaded content, and credits for online games. Companies in this subsection of mobile payments include Bango, BilltoMobile, Boku, and Zong.

Consumers Union, publisher of Consumer Reports, recently analyzed wireless carriers' customer contracts and found that consumer rights can vary widely between wireless carriers and the protections carriers claim to provide are often not spelled out in the contracts. CU has called on cell carriers to strengthen consumer protections in their contracts so that customers know their rights when things go wrong.

And things do go wrong. Non-delivery of paid-for merchandise is listed on Boku's online help center as one of the "most popular" troubleshooting topics. More than 181,000 and 63,000 visitors have found the need to query about, respectively, non-delivery and "still haven't received" problems.

We sent 23 Consumer Reports secret shoppers based all over the country to quiz their carrier's customer service reps about how to resolve a variety of potential billing problems. Based on the cell services to which our shoppers subscribe, we surveyed 14 customer service reps from Verizon, five from AT&T, and two each from Sprint and T-Mobile. In addition, a reporter asked each company's corporate headquarters the same questions, and we compared the answers.

Our shoppers said they were planning to purchase Facebook credits—a virtual currency used to buy items related to online games like It Girl, Mafia Wars, and Happy Island—from Zong, and they expressed concern about four what-if problems.

Our test results show that, on average, cell carriers' customer service representatives are not aware of their company's policies. Overall, the reps at AT&T gave correct guidance only 61 percent of the time, followed by Sprint, 50 percent, and Verizon, 38. T-Mobile, which last summer touted its Direct Carrier Billing, as convenient and "hassle-free," got the worst average score in our test: 25.

If the cellular carriers' official representatives don't know or can't accurately explain their companies' mobile payment protections to customers, then customers are more likely to have problems obtaining any remedy to which they're contractually entitled. The specific responses that customer service reps gave demonstrate the confusion. Take Verizon's responses in explaining whether Facebook credit buyers could undo their purchase and get a refund if they changed their mind after buying and how they should go about getting that done. The official correct answer we got from Verizon's corporate headquarters is: Call Verizon customer service, which should direct you to call the merchant, Zong, which must "mutually agree" with the customer to cancel the purchase.

More than 70 percent of the Verizon customer service reps got the answer to that question right. But some of those said they weren't sure or had never heard of Zong, leaving us with the feeling that they might have made a lucky guess. Meanwhile, other Verizon reps erroneously said, "you cannot get your money back," the purchase is "nonrefundable," or that the rep herself could refund the purchase.

At AT&T, where the correct answer from headquarters was that either AT&T or Zong could issue a refund, 67 percent of the reps answered correctly. But at Sprint (correct answer: Sprint or Zong can refund the money), only one of the two reps got it right.

At T-Mobile, the correct answer is: T-Mobile can issue a refund. One of two secret shoppers who called T-Mobile was given an incorrect answer. Our other shopper had to go through three reps before finding one that had heard of Zong. That rep said T-Mobile would "work with" the customer, but she was "not certain" a refund would be possible.

The other questions involved fixing an incorrect amount of Facebook credits purchased, double-billing, the right to dispute unauthorized charges, and where to find the carrier's mobile payment policies in writing.

At the moment, your exposure to such trouble is somewhat limited, since you can only bill "digital goods" to your cell phone. And carriers and mobile payment processors tend to limit how much you can charge—at BilltoMobile the monthly max is $25 for Sprint and Verizon customers and $100 for AT&T and T-Mobile subscribers. But consumers still deserve a clear, accurate, and stronger statement of rights.

—Jeff Blyskal