Complex sentence: AT&T Mobility is the second largest wireless telecommunications provider in the United States and Puerto Rico, behind Verizon Wireless, which has 111.3 million customers as of the third quarter of 2012.
Split into simpler sentences: AT&T Mobility is the second largest wireless telecommunications provider in the United States and Puerto Rico. Behind Verizon Wireless, which has 111.3 million customers as of the third quarter of 2012.