Reducing the amount of power needed to carry a conversation through the airwaves, made it possible for manufacturers to produce much smaller phones. While solid state circuitry advances have played an important role in reducing the size and weight of portable phones, the reduction of power requirements eliminated the need for bulky, heavy batteries to power handsets.
The first cellular telephone network was an analog system called AMPS. An AMPS is an analog, frequency modulation scheme (the same scheme used in FM radio). At the time cellular was introduced, FM modulation/demodulation circuitry was a commodity product, and so this was the most cost effective way to introduce cellular service. Digital cellular did not become cost effective until the 1990s when DSP based platforms came down in price enough to make it practical to create an all-digital telephone handset.

A cell phone has become easily accessible today, as it can fit in most pockets. A decent cell-phone will be available starting Rs. 5000/- and of course, the sky is the limit if you want an expensive model. Another thing to note is that cell phone prices are going down as the market becomes more competitive. At least where the cell phones go. The new tiny single-chassis phones are called “candy bar” models, a reference to their shape. Many weigh less than 200 grams, battery included. The familiar squat flip-phones — those that open like communicators from the old “Star Trek” — also are ultra-light.
Batteries are getting smaller too.