>>
the carrier's faultAs I said the behaviour is present in other operators too. I've asked a friends on Idea and Vodafone too(with Android). I just called a friend(before writing this comment) and he confirms that he is not facing this issue on Idea+iPhone5(he faced the issue on Nexus S with Airtel and then Idea too).
I had asked a friend with a Lumia and on Airtel and he said he doesn't have to do this.
>>carrier sends usage alert when you "turn off" data + bad connection that keeps..
No, it sends whenever there's 3G transaction and the usage period ends. I've observed that when I am using browser, I usually get the alert after I close the browser.
Airtel technical team(I was told they were the tech team) also confirmed that it is based upon usage and usually many apps/features rely for background syncing/net-access solely upon 3G/data so that is what makes the number of pop-ups too much. They also confirmed that it happens even when the phone is continuously on WiFi(I concur).
>>There are a few apps that convert USSD
Tried. Actually the most famous one is developed by an Indian guy(makes me think he was a victim himself). Didn't work.
Planning to root it. Google will push probably the last upgrade with 5.0(if at all). Will see if there's any ROM that handles it.