Cell phone towers were originally built by one company for their equipment only.

So just because there is a tower nearby doesn't mean it is for the company that handles your phone calls.

Many cell towers have been bought out and now lease space for the cell equipment, sometimes for more than one company.

But that still doesn't mean that all nearby cell calls are handled by that tower.

Only the companies that pay for their equipment to be on the tower can use it.
JD2
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T-Mobile USA to Sell Towers to Crown Castle for $2.4B ...

www.bloomberg.com/.../crown-castle-to-buy-t-mobile-u...
Bloomberg L.P.
Sep 28, 2012 - Deutsche Telekom AG's T-Mobile USA agreed to sell the rights to operate ... exclusive rights to lease and operate the antenna towers for about 28 years, ... while allowing T- Mobile to rent back antenna space on the towers.


AT&T confirms deal with Crown Castle to sell and lease cell ...

www.androidcentral.com/att-confirms-deal-crown-castle...
Android Central
Oct 21, 2013 - 600 towers sold and 9,100 leased with the option to buy .... People don't want ugly cell towers in their back yards, so if you have a tower already ...