Builder not giving possession of my flat

Hi,
I had bought an underconstruction flat with a reputed builder about 2 years back making full payment (thru bank loan). During booking, I was promised an year's time for possession. Slowly, the timelines kept on extending. 2 years have already passed. Meanwhile, the builder has already given possession to other wings in the complex.

Whenever I call up their office, no one picks it up. Even if someone picks, I am asked to wait on the call and after 30 minutes the call gets disconnected. For over one year I have been struggling just to get in touch with them. I am out of Mumbai now and hence even meeting them in person is not possible.

My wife was abused by the VP Sales for asking an update on the possession dates. Also he says that our wing will not be given till next year. Rates have gone up since the time we have bought. So this VP is asking my wife to cancel the booking and get lost.

Can anyone please tell me how to fight this menace? What is the legal way to counter this problem? Is the builder bound by any time period for delivery? My agreement says 2 years from the time of registration. Do I send a legal notice or do I send a normal notice from my side? Any legal help?

Those who have countered such a problem may please help.

Regards