every internet session you will receive a service message regarding the amount of data used.....
last 2 years idea customers get services message like test message...
now service message is changed like airtel...
Is it new? I was getting notifications after every data session even much ago, 5 or 6 months may be.
But, you cant check your remaining data in idle mode, only after each session, you will get it.
No one giving internet on national roaming yet you need to recharge special terrif.[only for Airtel vodafone and idea. Because other gives very low connectivity]