Have a read at this link from Offcom.
https://www.ofcom.org.uk/about-ofcom/latest/features-and-news/new-protections-stamp-out-switching-fr...
I suggest you raise a complain via the form on that page, and use the advice given on that page. For convenience I am copying it here:
----------------------------------------------------------------
New protections to stamp out landline switching frustrations
08 July 2019
You will have more power over your landline provider if it tries to stop you taking your phone number when you switch, thanks to a new process in effect from today.
People and businesses have the right to keep their telephone numbers, regardless of which phone company they are with.
Under Ofcom rules, phone companies must allow customers to take their telephone number with them to a new provider when they switch. This is known as ‘number porting’.
However, there have been times when providers made it difficult for customers – particularly small businesses – to keep their phone numbers when switching.
We have previously fined firms that have blocked customers from taking their numbers to a new provider, and forced them to pay compensation. Last year we told the telecoms industry to develop a process to override attempts to prevent customers from taking their number with them.
That new process is now in place.
How the process works
From today, if your request to port your number is being frustrated, your old provider will be put on notice and have up to five days to resolve any issues. If this doesn’t happen, you can now trigger a process that will enable your new provider to override this obstacle.
First, you need to submit a complaint on Ofcom’s website. This will generate a reference number, which you then need to give to your new provider.
An independent industry panel – which includes the Office of the Telecoms Adjudicator – will then assess your case. Subject to your case satisfying certain criteria, this panel can authorise your new provider to override the issue and expedite the porting of your number.
Protecting customers
Ofcom’s number porting rules protect people from the inconvenience and cost of having to change their phone number.
It is particularly important for businesses, who would face significant costs and other issues if they had to change their phone number, such as needing to change their marketing materials, or potentially losing business from customers who try to call their old number.
The rules also make it easier for people to switch. If a customer experiences problems when trying to port their number, this might put them off switching provider in future.
