As Yogi said, your phone must be Tri-band unlocked GSM phone.

I recommend Airtel as the all India coverage is very good. But Vodaphone will also do the job. You need to bring a passport size photo for the connection and photo copies of your passport. Check www.airtel.in for documentation requirements.

Depending on your usage, you could decide whether to use public phones or a mobile phone. Generally a mobile sim would be difficult to procure due to govt regulations on identification/ verification requirements. All cities of the country have manned public phones at every corner and these are much cheaper/ have the same cost as calls from mobile phones. The operator will assist you in making calls. Is a better option if you are in the country for a few days or weeks.

I would have to carry a mobile as will be leaving the city end of dec to some other places of India, mostly north-east india. Is that difficult to get a sim card?

I've check the lines, we are using 900/ 1800 as well, it's just difference as '2 carries" in india, and "3 carries" in malaysia, which i have no idea what is it about...

Regarding the map, the price online would be INR290, is that the same place i could get in the city? What kind of shops would available for that map? book shop probably?

Thanks