(AP) -- T-Mobile USA, the fourth-largest wireless carrier in the country, says it added a net 325,000 subscribers in the second quarter, with most of them choosing to prepay rather than sign two-year contracts.

The news cements this year's trend in wireless: The two biggest, and AT&T Inc., are attracting the valuable contract-signing customers, while Sprint Nextel Corp. and USA are competing with smaller carriers for lower-profit prepaying customers.

T-Mobile USA, a subsidiary of Germany's Deutsche Telekom AG, on Thursday said it ended the quarter with 33.5 million customers. Of the new customers, only 17 percent were on contracts.

Revenue fell to $5.34 billion from $5.47 billion a year ago, as both contract and prepaid customers paid less every month.

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