Airtel Broadband Connection — False Item calls on my bill

Respected sir,

I am using Airtel Broadband connection, No. [protected] where I disconnected my instrument and using only Internet.But In the Bill[protected] stated as Call charges done on my no. as 620.
While I called their service, they told its from your number and they are not accepting it as someone misusing my number through their exchange.

Even I tried to lock my number, still they have problem.

Could you please help me out as they are harassing me to pay for false call allocated on my number.

Thanks
Nimish