Your cell phone is a radio. The frequency used is primarily line of sight. If you want to increase the range and improve the reception, raise the height of the antenna.
In a car, install an external antenna.
And, why I haven't seen this yet, I don't know...A "base station" for your cell phone. ie: Home/Roof mounted antenna, plugged into a "base" which charges/powers your cell phone.
For fringe/rural areas, this might be ideal...one phone!