By Aishwarya Venugopal

(Reuters) - Sprint Corp, the No. 4 U.S. wireless carrier, estimated quarterly operating revenue above analysts' expectations as heavy promotions helped attract more postpaid customers.

The company, majority owned by Japan's Softbank Group Corp, said on Tuesday that postpaid phone subscriber additions doubled to 347,000 in the second quarter from the prior quarter.

But the company's shares, which touched more than two-year high earlier, reversed course to trade down 3.3 percent at $6.68. The stock had nearly doubled this year through Monday's close.

CFRA Research analyst Angelo Zino attributed the stock's fall to Sprint missing adjusted EBITDA expectations.

Sprint estimated adjusted earnings before interest, tax, depreciation and amortization (EBITDA) of $2.35 billion for the quarter, slightly below the Street's expectation of $2.4 billion.

This would be the company's first EBITDA miss in six quarters, Zino said, citing aggressive promotions and new launches.

Sprint, along with rivals AT&T Inc, Verizon Communications Inc and T-Mobile US Inc, offered a free iPhone 7 to customers turning in recent iPhone models.

Sprint said in September demand for iPhone 7 was strong, with pre-orders nearly four times higher than a year earlier. T-Mobile US Inc also reported a similar rise in pre-orders.

Postpaid subscribers are customers who pay their bills on a monthly basis.

The company also estimated a postpaid phone churn, or the rate at which subscribers defect to other networks, of 1.37 percent for the second quarter ended Sept. 30, a slight improvement from the prior quarter's 1.39 percent.

Operating revenue rose 3 percent to $8.25 billion in the second quarter, beating the average analyst estimate of $8.02 billion, according to Thomson Reuters I/B/E/S.

Sprint's net loss fell 75.7 percent to $142 million.

The latest quarter included a non-cash after-tax gain of $218 million related to certain spectrum swaps with other carriers, the company said.

Sprint is scheduled to release results on Oct. 25.

(Reporting by Aishwarya Venugopal in Bengaluru; Editing by Anil D'Silva and Sriraj Kalluvila)