Sprint today became the last of the three major U.S. iPhone carriers to report earnings for the fourth quarter of 2011, disclosing that it sold a total of 1.8 million iPhones during the quarter. While well below the 7.6 million activations reported by AT&T and the 4.3 million activations reported by Verizon , Sprint's iPhone surge helped it achieve a new record high subscriber base of 55 million customers.