In my (very) limited experience, a number-porting issue can arise because of, for example, a mismatch in the data held by the systems at BT Wholesale and Openreach: my understanding is that they have separate systems for holding details of physical lines and numbers. If such a data mismatch occurs during a landline / number port (for whatever reason), it can require a concerted effort by all concerned to resolve it.
In my case last year, it was my LCP (losing comms provider) that had to raise a case with both Ofcom and BT Wholesale to resolve my particular issue – at the time my GCP (gaining comms provider) had little visibility into the issue.
In your case, things may potentially be tricksy because your LCP is BT and your GCP may be comparatively inexperienced at handling such issues: I would suggest you politely ask your GCP to consider whether they need to raise cases with BT Wholesale /Openreach and / or Ofcom to get the issue resolved.
Landline / number porting can be complex, and is not always a straightforward process: I appreciate the new One Touch Switch framework from Ofcom is meant to make things a lot simpler for the customer, but at the end of the day, there is still a lot of technical work potentially to be done in the background, simply because some of these systems at BT / Openreach have a lot of history and baggage to them: the next year or two will I’m sure, highlight any number of issues with the vintage systems and data still in place. It will I think need a fair bit of co-operative work and patience to resolve them.
If you want to know more, there’s lots on the web to read about the process of number porting. For example, the following may well be out of date by now, and might be considered overly detailed by some, but it gives a taste of what’s historically been involved in porting a number:
https://www.magrathea-telecom.co.uk/industry_porting...
Best of luck