Hello, my name is Jim and I am a qualified lawyer happy to help you today.
You can allege breach of contract (due to unfair cancellation charges) against the phone company (you have consumer rights here which have been infringed in my view).
Given the circumstances of your case I recommend that you make a formal complaint to the company and if that does not resolve the matter ask them for a final response letter (a "deadlock" letter).
The complaints procedure should be available online (let me know which provider you are with and I can check for you).
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 the phone company which would hopefully result in a cancellation of the bill. They can also order the phone company to make a financial award if they have acted badly.
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 the company's conduct.
You can escalate to the adjudicator here : https://www.cedr.com/consumer/cisas
If you have lost any money then you could sue the phone company but the court may consider that to be premature if you do not go through CISAS first of all.
You can sue for your losses via the http://www.moneyclaim.gov.uk website which is easy to use and no lawyer would be required. You can tell the debt collectors that you dispute the bill and that the phone company will have to sue in the small claims court. It is easy to defend a small claim (no lawyer is needed) and I can help you do that when the time comes.
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 Q&A's at no extra charge and Just Answer will credit me for helping you today.