I absolutely agree with what You are experiencing -- same in my case, where I paid the Final Agreed Payment on April 8th, only to be greeted with more monthly accounts -- and this is for an account number that Vodafone say no longer exists!
The problem is that people in Call Centres Will Not, or Do Not take responsibility for what they tell you.
I say: take me to Court for Non Payment; at least a Judge will be able to talk to You and sort this out.
Then : Why Vodafone; why would you even consider it --- Useless Tech, and Even more useless Customer Service, by Call Centre People who can not Understand or Talk Proper English. Pay peanuts, and You get monkeys -- That is what has happened becauise V.F wants to get things at the cheapest rate -- Large Corporate Greed -- Exactly.