Just found this
article According to it Airtel is violating TRAI norms by charging Re 1/incoming SMS on roaming.
Check this pic from the article. Airtel has clearly specified that incoming sms will be charged.
And now check the present image.
How cleverly they have modified the table by hiding the info. Now they can continue charging for incoming SMS which you obviously will miss here. And that is how you violate a TRAI mandate folks.