Possible actions after notice to pay bills incorrectly generated

I had filed for the disconnection of my postpaid SIM after making all due payments. Post this, Vodafone disconnected but after a week again reconnected the SIM without any intimation or request from my side. They have been continuously generating bills month after month even though the SIM is non functional and cannot make or receive any outbound or inbound calls. They have now sent me a legal notice asking for the payment of the bill. I am not ready to pay the bill since there was no usage and the connection was reconnected without any request from my side. Completely the fault of Vodafone. Kindly suggest how I should respond to the notice (received on email). What will be the consequences of not responding to the notice?