FTTC Number port to VOIP , keep FTTC service on new number
Hi there.
First of all thanks for your loyalty and welcome to our community forums.
Unfortunately what you're wanting to do may be tricky, because if you port your number to a VoIP provider, then ultimately the line we're providing will cease and your broadband service along with it.
If we instead first renumber your line to a different number, then the number you want would cease and I believe that the number will need to be active in order for you to port it to a VoIP provider.
While not ideal* a workaround may be to port your number out, allowing your services to cease and account to close, then sign up a new account at http://www.plus.net where we should be able to simply reactivate the recently ceased phone line usually within 1 working day, then place your fibre broadband back on the line although this does take a 5 working days depending on engineer availability too.
*Two possible issues I see here is the inevitable downtime you'll experience, also in the interim period between the line ceasing/reactivating and placing the fibre order, there may not be any capacity at the cabinet left, i.e. the port you were on may be taken up by another line. (Got first hand experience regarding that last point with my own line a few years ago)
Or, sign up a new account before the old account ceases, installing a new line/fibre service at your address then once that's up and running, port your number to VoIP ceasing the previous line and service as part of the process.
Though this option will incur a £49.99 line installation fee plus there may be some overlap where you're paying for two lines at the same time. That said there are no spare pairs/lines showing at your address so it's likely installing a second line will go to a survey and a lot of engineering work needed which may take weeks or months.
Sorry for the long post and that there doesn't seem to be a straightforward solution, unless VoIP providers can use ceased numbers? It may be worth contacting one to ask.
Feel free to let me know the above doesn't make sense or if you have any questions etc.
@Gandalf I believe there is a 'Renumber with Number Export' option somewhere. This https://www.revk.uk/2016/04/number-porting-broadband-and-voip.html is interesting reading when/if you've got a few spare moments (about when the next blue moon occurs I guess) 
No worries.
Last night shortly after your post I emailed the suppliers of the WLR system we use(Which is essentially a layer over the Openreach systems) and not long after I received a reply back advising the functionality isn't built into their systems due to a lack of demand, but they should be able to fulfill requests for this on an ad-hoc basis, placing the "Renumber with Number Export" themselves.
@Earlyadopter, if you would like to go ahead with this option feel free to let me know and I'll be happy to help, though from reading the article and from what I've been advised by our suppliers, there is a chance that things may go wrong and the line ceases anyway.
@Gandalf above and beyond the call there
and from what I've been advised by our suppliers, there is a chance that things may go wrong and the line ceases anyway.
sounds about par for the course where BTw & OR are concerned
I suspect it might be difficult to correlate the renumber order with the order to transfer the 'exported' number. Probably easier for the likes of A & A where they are placing both orders. In @Earlyadopter s case the transfer order will be to another provider.
If I were him, I'd stick with status quo...
Yeah it sounds similar to when we'd place a simultaneous phone and broadband order, where we're just doing the broadband side of things and the phone order is placed by another provider. Both us and the other provider would need to use the same LORN (Linked Order Reference Number) or the order will fail.