Airtel's billing system: Worst possible?

Here's what happens 2 months in a row...
> Airtel alerts me to pay by due date for my landline/broadband connection
> I go to www.airtel.in and dutifuly pay through my hdfc credit card
> Airtel displays on the site in red that payment was unsuccessful
> At the same time, I get an SMS from HDFC which says (verbatim): Thank you for using your HDFC bank credit card for 996.00 in BHARTI AIRTEL LTD - B& @ 10-Apr-2009 10:46 AM
> I get 2 more alerts from Airtel on next 2 days which say (verbatim): The payment for your Airtel fixedline bill is due on 13-APR-09. Kindly make the payment of Rs. 996 to avoid the late fee charges. Pls ignore if paid.
> I call up Airtel customer care, and am blatantly told that you last made a payment last month, and this month's payment has not come in yet. I told them about the HDFC message, and I was asked to call in 2 days to verify again (2 days would have meant after due date)
> I then call up HDFC customer care, and am reassured that payment has been made to Airtel already, and am also told that this happens very frequently with Airtel.

Given that airtel has paid IBM billions of dollars for their IT/tech support, i am surprised why this minor problem persists every month? Why can't airtel put its systems in order to avoid giving agony to its "valued" customers time and again?