This used to matter more, but these days, unless you're dropping a tower where no one else has coverage you simply lease a spot 5-10 feet down on a pole your competitor uses and light up a new coverage spot.
For example. This tower in not in sussex, but a town close to you.
Note all 4 major carriers have coverage on that. But if one didn't, they wouldn't need to go through the headache of putting up a new tower.
(as an aside, i wish all carriers had a site like tmobile does showing the towers they own and what carriers are on them. Also note these are the towers tmobile owns, they certainly have antennas on other towers that are not theirs, as do all other carriers)