Airtel — airtel has converted my postpaid number to prepaid without my knowledge

I have been using below number in airtel post paid plan for more than 3 yeas. Suddenly both the numbers are showing in prepaid plan.
My numbers are,
[protected]

It is taken under corporate plan.

I received last month bill for my number and i am not yet paid. How can you convert my number to prepaid with letting me know and my knowledge.

I need to resolve this asap.