Airtel 3G is not activated on my SIM- what to do ?

I had just send an sms to 121 to activate 3g ...but it says 3G is not active on your airtel

You don't have to activate 3G as now its just activated by default and to use it you have to make sure you are using 3G network and also have 3G pack. (if you have 2G data back then you will receive 2G speeds even over 3G network).

3 G service automatically activate once you take any recharge. No need to seperatly activate 3G by SMS