NSString *tenDigitNumber = @"5554449999";
tenDigitNumber = [tenDigitNumber stringByReplacingOccurrencesOfString:@"(\\d{3})(\\d{3})(\\d{4})"
                                                           withString:@"$1-$2-$3"
                                                              options:NSRegularExpressionSearch
                                                                range:NSMakeRange(0, [tenDigitNumber length])];
NSLog(@"%@", tenDigitNumber);

此为格式化334的形式,具体的其他格式可以修改正则表达式.