Negative Balance — how can my balance be negative in vodafone prepaid connections?

Address:

New Delhi, Delhi, 110016

I am a researcher in iit delhi, we use our wifi connections, but sometime when we go outside then use my data, i am a regular customer and every month i pay the 1 gb internet service will voice call in my prepaid connections so my question is how can my balance is negative?
I raised the question with customer care but the cruel lady and man use rough language so i decided to port my number from vodafone to jio, because we do not have time to discuss or fight with such nonsense persons.
Thanks
Vodafone,
Yogash chandra joshi
[protected]
Iit delhi.