Shares of Sprint Nextel (S) soared as much as 16% on Thursday following better-than-expected sales and an improved adjusted full-year operating outlook despite a drop in total customers and weaker earnings.

The No. 3 mobile operator said operating revenue climbed 6% to $8.8 billion from $8.3 billion a year ago, and topped the $8.73 billion forecast on average by analysts in a Thomson Reuters poll.

The Overland Park, Kan.-based company grew wireless post-paid average revenue per user to $60.88 from $56.67 last year and said wireless retail service revenues climbed 7% to $7.2 billion.

Sprint attributed the gains to strong sales of Apple’s (AAPL) iPhone, which made up 40% of new post-paid customers. That stands out because iPhone sales for some of its competitors dropped last quarter.

Post-paid churn declined to 1.69% -- the company’s best-ever rate -- from 1.72% a year ago and the carrier said that 60% of the customers leaving its Nextel push-to-talk platform plan to stay at Sprit, a growing percentage from 27% a year ago and just 46% last quarter.

Sprint lifted its fiscal 2012 adjusted operating income outlook to a range of $4.5 billion to $4.6 billion from an earlier view of $3.7 billion to $3.9 billion and said it expects consolidated net service revenue growth this year of 4% to 6%.

However, the carrier widened its quarterly loss by more than expected to $1.4 billion, or 46 cents a share, from a year-earlier loss of $847 million, or 28 cents. Analysts had been looking for a loss of just 40 cents.

Earnings were impacted by about 39 cents largely from costs related to the shutting down of its Nextel network as well as an impairment charge related to an investment in Clearwire (CLWR).

A sharp drop in total customers, mostly related to Nextel, and a worse-than-expected loss of 246,000 two-year contracts, also hurt earnings. While Sprint gained 442,000 new customers to its namesake brand, that was more than offset by a drop of 688,000 in the Nextel unit.

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