Full advance made but till now product not delivered.

Location/place: Bangalore

Name of company/service: Datawind

I have a made the payment in march and they promised me to deliver the tab by April, but later without informing me they delayed it to 1 month and later to that 1 more month. When ever i call them they just make useless excuses and disconnect the phone. Later they had reply to my mail complaint stating that i will get my tab by 25th June but till now no sign of delivery. When i called them they say its been delayed by 15 more days and disconnected the call.
Now i want to take it legally.