Seek compensation for wrongful levy of the charge

I had international roaming service enabled on my Airtel connection couple days before travelling to Abroad. Later requested to activate 1-day IR pack. Due to time-zone difference, Provider is saying the plan was activated on 8th March and the usage has been made on 7th March. Basically they say - Packs and plans charging is applicable as per Indian standard time. But they are considering IST at the time of activation and EST timings for billing the usage. Either way they should consider converting to IST or reflect the timings in EST. Owing to all the above, I have been chased to pay the bill(Only voice) by waving off the data charges. Due to pending of the payment, Services are barred on my connection. I gave multiple visits to store and spoke to Nodal/Appellant officer, No luck. I have even proved by providing detailed mail on flight/PNR(Which luckily was on 7th March as per IST), Justifying i was on flight for the timings they say the usage was found. I'm tired of chasing/proving my innocence with them and would like to send a legal notice before moving to new connection with new provider.