Why red light in the cell phone tower?

Answer 1

The flashing light - is a warning to aircraft at night. It shows the pilot how tall the tower is, to allow him to avoid hitting it. That's especially useful for phone towers close to airfields, where planes are at much lower altitudes than usual.

Answer 2

Cell Phone towers and any other tall objects including buildings have a blinking red light on the top to warn low flying Aircraft that there is an object in its path.