Crown Castle International (NYSE: CCI) is the largest cell phone tower company in the U.S. It leases antenna space on its over 24,000 towers to wireless service providers, including Verizon Communications (VZ), Sprint Nextel (S), T-Mobile, Alltel, and AT&T (T).^[1] In light of its 2006 merger with Global Signal, the company is has slightly more towers than its top competitor, American Tower (AMT).

Cell phone towers (often called cell sites since there are both tower and rooftop sites) derive revenue from multiple tenants who use the infrastructure simultaneously. Because of the regulatory difficulty of building new towers (at least domestically), growth is largely driven by adding tenants to existing towers. This gives the tower companies significant barriers to entry and strong cash flow from incremental business at each tower.

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