Sprint Nextel Corp. posted second-quarter profit margins well below Wall Street estimates, sending its shares down 16 percent as the No. 3 U.S. mobile provider lost more subscribers than expected, Reuters reported.

Investors also questioned whether Sprint would meet its financial and subscriber targets for the full year after the weak results. Sprint’s operating profit margin of 16.3 percent compared poorly with Wall Street expectations of around 19 percent.

Sprint was unable to hold some customers during the first full quarter when Verizon Wireless sold the Apple Inc. iPhone and AT&T Inc. sold a discounted iPhone.