Airtel — misguidance about porting of sim

I port my aircel number to airtel in september. I want to convert to airtel prepaid but they told that first three months sim will be postpaid. They charged me rs 500 and informed me that i need not to pay any bill if any bill was prepared.
In november they send a bill of rs 360/. When i did not pay as i was instructed they barred the network services. Then i had to paid the bill. And now again they have sent a bill of rs 424/
What is benefits of porting of sim? I was totally misguided.