How do you increase the signal of a mobile phone?

There is usually no easy way to increase the signal of a cellular phone, although some models have better range than other models. One possible way to increase the strength when you are in a car is to have a "car-kit" which boosts the signal strength and uses an external antenna (on the outside of a car). If this is a critical concern for you, you can probably avail yourself to a "transportable" phone, which operates at a higher power than a standard cell phone, but these are also several times larger and heavier than a standard phone and really only a solution for those who must be able to make calls in fringe areas.

Other tips that will improve your signal is to use the phone outside when possible, the range will be significantly decreased if you are inside of a (metal) car or inside a building. Extend your phone's antenna if that is possible on your phone and hold the phone as vertical as you can. There are always some "dead" areas where cellular signals are weak, obstructed or have significant interferrence, so moving a few meters will sometimes make for a much clearer call. If you have seen the clear stickers with a gold foil "antenna" design and claim to increase your cellular signal, they are a waste of money and have no plausible basis on why you could expect them to help.