There are phone numbers know as "BT" numbers, (possibly issued by Openreach to BT Wholesale but that is simply a supposition by me), and there are the numbers issued by LLU CPs such as Sky and TalkTalk. This gives three ranges of numbers. Vodafone may have a fourth and there may be more.
You may remember many ISPs would say you had to have a "BT" line to have broadband with them.
Until recently if you entered an LLU number into the BT Wholesale estimate checker
it would simply not recognise it as existing. This may in fact still be the case, but as I have a "BT number" but not with BT Retail I can't check.
It was not possible to port an LLU number to the BT system so that BT Wholesale-based ISPs could take it over. But if you started with a BT number, then moved to LLU, you could port it back. (Whilst ported to LLU the checker linked above still would recognise it).
You say you get the impression from Ofcom that you should be able to transfer it. I believe you are correct, as recently Ofcom made this a requirement. But I could be mistaken and it hasn't yet been implemented. It's late and I'm tired and trawling through Ofcom output can be piggish! However, I have a feeling it is somewhere in this document
If it isn't, if you search on the Ofcom website, which clearly you have been looking at about this, you should be able to find it.
It's possible the Plusnet adviser you spoke to isn't aware of this change, or it's possible Plusnet systems have not yet been changed to be able to port LLU numbers to themselves. All I can suggest for now is that you keep chasing this down.
Kindness isn't going to cure the world of all its awfulness but it's a good place to begin. Daisy Ridley.
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