Ok, first thing is that 5 to 10 watts in the cell phone band is a MASSIVE signal.
Your in India and I'm unfamiliar with how they deal with interference there, but in Sunny and occasionally wet Australia we have ways of dealing with such trouble when it occurs.
Don't push him Dave else we'll be importing them from India next week. You laughed at the chinese last century and now they build the base stations !
The first thing to consider is ..... damn... there's so much to consider. You're biting off a lot more than you can chew.
Ok... as mentioned, a passive repeater system is much simpler and just as effective.
Using high gain antennas is the best choice.
If it's just an indoors problem, then you're much better just finding a car cradle kit for the phone and use a high gain external antenna like a yagi.
800-960 and 1800-1950 is a better way to describe them.
Recycling old mobile phone tower equipment is a place to start and learn. If you can find an old TMA then you're 80% of the way there if you really must attempt it.