Legal notice from mobile service provider

Dear Sir/Madam please guide me what to do? I was using a airtel post paid number for 10 months in which I was given a 'x' number free for call from anywhere. suddenly in 11 th month they made that 'X' number charged for roaming outgoing and I got a huge amount bill but I refused to pay the same. The number I was calling every day and talking for hours for 10 months and as it was free the 'x' number was not reflecting even in my monthly bill. service provider company deactivated my phone number, again an executive called me and said just pay half the amount and we will make you number active and give remaining half as a discount to me and immediately I paid the half amount but the number remained deactivated (I am having call recording with executive also bank details as paid the half amount immediately and bill details) now its 2000 rs amount unbilled so service provider has send me legal notice for 2000 rs what should I do.