There are many areas of the country where cell phones still do not work, link building services even with amplifiers. You may not be able to do anything about this except buy a very expensive satellite phone. Many other areas have marginal coverage. There may be a signal in the air but your phone is barely able to pick it up due to a number of reasons.

Modern cellular phones have a very low power output, about a third of one watt or less, and a very puny little antenna that actually reduces that one third of a watt to even less.
With a properly sized external antenna (outside on your vehicle or home’s roof) you can boost the signal from your phone several DB or decibels, which equates to several times your phones output power. information technology service provider

In addition you can add to that antenna an amplifier that boosts your phones puny wattage up to the maximum wattage allowed by the FCC, three full watts.

In the past many cell phones were made with a small antenna jack, on the back or side of the phone, under a rubber plug, that you could connect to a rooftop magnetic mount antenna.
The problem nowadays is that many new high tech phones do not have these external antenna ports.

In that case your options are using a stick on “passive” adapter which conducts the signal from your cell phone’s internal antenna into the wire leading to the external antenna.

These are notoriously “lossy” and do not conduct much of the signal into the outside antenna.
The other alternative is to use a wireless repeater type of cell phone amplifier such as those made by the industry leaders, Digital Antenna and Wilson. Alfao

This type of cell phone amplifier works like a small cell phone tower and receives the signal that is transmitted from your phone, amplifies it many times, and sends it out to a high gain antenna that also amplifies it further. It works the same in reverse, amplifying incoming calls many times over so that you will have several bars on your phone where you may only have one.

While these devices will not perform miracles, such as allowing you to make a phone call in the bottom of a canyon far away from any tower, they will allow you to use your phone in marginal rural and urban areas where your phone is not strong enough on its own.

I have used one for years as a long haul trucker and have been able to make emergency calls in areas where other cell phones do not work.  Cellboost

The other nice feature of the repeater type amplifiers is that they will allow any cell phone, or even multiple cell phones and even computer internet aircards to be amplified if they are within a few feet of the small antenna inside of your vehicle or home. Most units are 12 volt DC and can be placed on an AC adapter for use in the home. Other models are made only for home use.