The US mobile operators, especially AT&T, have been making concerted efforts to disrupt the traditional towercos, aiming to reduce their costs ahead of densification and 5G (Verizon has set a target of slashing operating costs by $10bn over four years, in which new approaches to sites is one tactic). Last fall, Verizon and AT&T announced a highly unusual joint agreement with a company called Tillman Infrastructure, which introduced both a new cell site provider, and a new approach to traditional site leasing, to the industry. AT&T followed that with an amended relationship with one of the big three towercos, Crown Castle; and another new tower partner, Cityswitch. Now, AT&T has come out with a bullish update on its side of…