If you don't pay VM's requested amounts or indeed stop the direct debit then they'll record that as a default under a credit agreement, and your credit history will be stained, and can then cause problems for the next six years with credit providers, mortgages, car finance, property rental, energy or telephone contracts. All this happens through IT systems, refusing to pay won't make anyone at VM pay any human attention to your underlying problem and complaint. Sounds like they're already treating this as overdue, and your credit history may already be in trouble. A quick check on Experian may clarify that.
Activating Volt benefits should not (on its own) create a new contract, so taking what you say at face value rather than trying to unpick the detail, I'd suggest you use the original complaint reference as the basis for an escalation to CISAS, the industry complaints adjudicator. Be clear on the story of the problem, the unrequested contract, unagreed new charges, failure to handle the complaint properly, and specify you want the matter resolving by the removal of the unagreed new contract, correction of the price to what it should be (?), compensation for the poor customer service, also for the poor complaint handling, also for the imposition of an unagreed new contract, and most importantly of all, for VM to clean up the credit history mess.
You might want to give VM staff here the chance to resolve the matter before you involve CISAS, as that's potentially quicker and easier for you. Sometimes the forum staff can get complicated things resolved for people, occasionally they can't or the company won't, and that's when CISAS is your only route forward.
