One thing to note is that you can listen to some cellular conversations with a police scanner..... But the conversations you can hear are only from analog cellular phones and not too many people use those today..... now days Everyone has digital cellular phones.....
Now these Scanners are availabe as Wireless comunication devices, Indian GOV and Department of Defence will b very angry once they find out u are running one

without licence

and getting one licence is tougher than getting a gov job
Now these Digital Phones uses encrypted connections... so one would need a divice to trap the frequency, then a computer with rigt software to decrypt it...
As far as I know it can be done with right budget, but in India u will ve violationg Privacy ACT and Intelectuall Property act, and some Corporate ACTs of the Network u are trapping... So 1st consult ur Lawyers
To do these, u will need pro help, and these guys do that,
http://www.iiiweb.net/go/contact get in touch with them with a right budget to see if they can help or not !!!!