Airtel - Tamilnadu — Wrong billing on GPRS usage

Let me start this in a politely way, I don't want show all my grievance at once

I have a POSTPAID connection from AIRTEL - Tamilnadu

I have been billed wrongly on my GPRS usage for the 3rd time, whenever I change my plan tariff, their stupid system cannot handle the changes properly

I have had a GPRS plan of Rs599 per month for 1GB free download, It means for a fixed monthly charge of Rs599, my usage can go upto 1024MB

Later on I changed my plan to Rs350 per month for 500MB free download, It means for a fixed monthly charge of Rs350, my usage can go upto 500MB

At the end of the billing cycle,
For first 9 days the GPRS plan was under Rs599 for 1GB monthly plan, for which they had charged Rs180 on pro-rata basis (my usage was only 53MB)
Then for next 21 days the GPRS usage was under Rs350 for 500MB monthly plan, for which they had charged Rs245 on pro-rata basis (my usage was only 167MB)

but looking at the breakup, I have been billed by Rs 1315 extra apart from the Monthly charges during my Rs350 for 500MB monthly plan (that is, they also charged money for ever KB usage)

Calling up the customer care, that guy doesn't know 1GB = ??? MB
No other option, I had to break my head for explaining problem for which he had a standard answer "Sir you have been billed on pro-rata basis"

Can anywhere in the world a PRO-RATA BASIS CHARGES would be greater than MONTHLY CHARGES.

I am struggling with the problem for last 5 days, explaining 3 to 4 guys but all of them have one standard "Sir you have been billed on pro-rata basis"

"THIS IS THE 3rd TIME I AM HAVING THE PROBLEM WITH AIRTEL GPRS PLAN CHANGE" please suggest me whether to CONTINUE their service or NOT

- Balaji [protected]