I presume that there is nothing wrong with taking FTTP with voice (such as the BT Retail product) but not using the voice component and porting your original number to a third party VOIP ?
Porting the landline number that's part of a bundled broadband/phone package will cease the broadband.
If you try port your Digital Voice number I believe that will cease your FTTP.
You need to arrange it in such a way that you order FTTP separate to your existing landline number.
Either take a data only FTTP service (no landline number) or order FTTP with a new number.
If you are currently on FTTC then order FTTP separately. Wait till the FTTP is live and then port the FTTC number to a VOIP provider.
Your current ISP sees this as just another migration and this will cease the FTTC line.
As mentioned OpenReach do offer a service to overcome this called "renumber with number export".
It lets you port the number to VOIP and issues a new number, keeping the broadband active.
I'm also only aware of AAISP offering this to customers though.
I don't believe BT use it.