Hello, my name is ***** ***** I am a qualified lawyer happy to help you today.
You can allege breach of contract against Vodafone (you have consumer rights here which have been infringed in my view). Debt collectors have no powers - they simply write to you to demand payment. They are not bailiffs so cannot enter your home. Just turn them away and tell them you dispute the charge with Vodafone.
Given the circumstances of your case I recommend that you make a formal complaint to Vodafone and if that does not resolve the matter ask them for a final response letter (a "deadlock" letter).
You can make the complaint with Vodafone here:
Once you have the deadlock letter the next step would be to escalate this to the adjudication scheme (CISAS) - they will investigate and liaise with Vodafone which would hopefully result in a refund of any monies paid. They can also order the phone company to make a financial award if they have acted poorly.
The service is covered by the Consumer Rights Act 2015 so you have a right to expect a reasonable service - if they do not provide that then the regulator will step in but it also means you can sue for any damages or losses incurred as a result of Vodafone's conduct.
You can escalate to the adjudicator here : https://www.cedr.com/consumer/cisas
If Vodafone issue a small claim, which is doubtful as it would cost them the same again to issue it and then more to instruct solicitors - ensure you fill out the response pack which the court will send you. You can always come back if required and I will assist.
I hope this helps - if you can please accept the answer and give me a 5 star rating (there should be a button at the top of your screen to do this), I can answer follow up questions at no extra charge and Just Answer will credit me for helping you today.