I am using Tata docomo same no. from last 7 years and I have no complain against it. If you are in Kolkata, Tata docomo would be one of the best TSP that you can get at affordable prices. One GB internet pack comes at around Rs.92/- and the speed is also good. You are getting Full talk-time at Rs.60/- recharge, what else do you want.
Other providers of telecom service like airtel provides full talktime at Rs.130/-. I don’t know about other places but network coverage in kolkata is very good. Also taking about the customer service, they are very helpful. I would personally prefer tata over other networks.