By Jennifer Saba
Verizon's perch as the No. 1 carrier in the United States looks secure.
The $200 billion telecom firm reported a rise in wireless subscribers on Thursday. But the gap is narrowing. Cable rivals Comcast and Charter are also offering mobile services, AT&T is trying to become a media company, and Sprint and T-Mobile may yet merge. A heavy debt load could hobble Verizon's ability to quickly change lanes.