After the leases expire, Crown Castle can buy the towers for about $4.2 billion. The average lease runs for about 28 years. It's not the first big tower purchase Crown Castle has made from wireless operators. In October 2012, the company sold $1.65 billion in debt to help finance a $2.4 billion purchase of about 7,200 towers from T-Mobile USA Inc. ( TMUS) Crown Castle said the towers involved in the AT&T deal are located in top markets across the U.S. It says nearly half the AT&T towers are in the nation's top 50 markets. The company has been focusing on the nation's top 100 markets, where it expects most wireless network activity and upgrades to occur. When the deal closes, Crown Castle will have about 40,000 towers through the U.S., keeping it the largest wireless infrastructure operator in the country, it said in a statement. The company has scheduled a conference call for 7:30 a.m. EDT Monday to discuss the transaction and its third-quarter results.