Don't overlook that your selection of cell phone can have a profound effect on your service area. A whole lot of small phones have essentially the same electrical components and capabilities but there are exceptions. Many carriers have tech support that will clue you into any phones they carry that have significantly greater operational range.

A friend of mine that work a lot in remote rural areas keeps his old 'handbag/brick' cell phone from the 90s around because it has significantly more transmit power and includes an easy to use antenna connection. He keep a variety of antennas on hand. A rubber pigtail that is rugged and good enough for in town, a 18" unit for semi-rural areas, and a 6' whip that allows for service in the sticks.

The tech department of the service provider will know what phones have better sensitivity and an antenna connection. They will also sell you a suitable antenna. Usually you can get the same units cheaper online.

There are also behavioral aspects. Gaining altitude, top of a hill or building, up a tree, or standing on top of you car can help. Avoiding ground clutter, buildings, trees, hills also helps.