Indeed you can't migrate when the cabinet is full despite you always keeping the same port. There's no way round this. Just keep checking for availability and order once it shows capacity exists.
Many years ago it wasn't uncommon to "lose" your port during an attempted migration when the cabinet was full. Back then ports were often physically changed during a migration.
The system would accept your order despite there being no free ports.
The system would also give away your existing port to the next person who ordered or to someone on the waiting list.
There is no additional risk of losing your telephone number because the cabinet is full of not.
Some providers simply don't support number migrations fully.
Vodafone are (or certainly were) 1 of the worst and try their best to convince you to renumber when migrating to them.
Some providers simply don't support migrating in numbers from LLU providers like Sky.
It all depends on your chosen providers porting agreements and who's number it was originally.
You will probably find it harder to migrate the number out from Sky than you did to migrate it in, with less choice if keeping the number is a necessity.
What a load of rubbish. So effectively once the cabinet is full you're basically stuck with a poor provider until such a point as you can get an order to go through, for no obvious reason? Is this stuff held together with wet string and sticky tape?
I can see I'm opening myself up to a world of pain getting involved in trying to migrate them.
EE can't migrate the number, and offer only ADSL.
Plusnet appear to be able to migrate back, but no offer of FTTC, just ADSL.
BT can migrate the number back. Interestingly, if you try to progress the ADSL order with BT, at some point in the process it realises you have an FTTC connection, and it removes all the ADSL options and just gives you the FTTC options, BUT it removes all the introductory offers - so no 6 months half price for you!
Wet string and sticky tape? Methinks that was generous. Peanut butter and the tears of small children are holding this lot together...