I don't know how much help an extra antenna would be. Cell phone signals are microwave and are pretty much line of sight transmissions. They are not like AM radio signals and they dont do the skywave or bounce off the stratosphere trick either.
There is a bit of follow to the curve of the earth, but it is not much.
I know one spot on a highway where there is clear reception for 1/4 mile. There is no reception at all for the 35 miles either side of that. It just happens to be where the topography allows line of sight to one tower which is a long distance away but blocks all the rest. Once you leave that 1/4 mile all the towers are below the line of sight because there is a range of hills between the highway and them.

So as a result of this line of sight signal trait you quite often can get a signal if you climb higher up a mountain or if you can get a sight line through a pass to where the towers are.

I thought it was a good article, though they obviously had to keep it pretty simple to fit the column.