No Communication unjustified billing

Our account was cancelled due to it not being used anymore. All accounts are are billed in advance and we have since received final demand letters for x2 months in arrears which I do not understand how it is possible as your accounts are billed in advance and it was a month to month agreement. Over and above we received another invoice for "Penalty Fee for Non Returned Hardware" which was purchased outright at the beginning of the agreement.

We are now being "handed over" for collections/legal action.

But the biggest issue is you cannot get through to anyone in accounts to resolve or query the issues. There is either no answer or the call cuts off with no call back.