Is it not posssible to just order a new FTTP broadband service which may come with its own digital voice service and then when that is working port the number from the old DSL service to a VOIP provider thereby terminating the DSL service?
Yes, but an awful lot of people aren't served by FTTP yet. And many of those with FTTC will see their landline service migrated over by their provider to that provider's own proprietary 'Digital Voice' / VOIP system before FTTP becomes available at their address.
So for those who want to port their landline number to a VOIP provider, at present they either need to:
(1) Get a new copper line installed for a new DSL broadband connection, then port the old landline number to a VOIP provider which will in turn cease any associated broadband service on the old line, or...
(2) Go through the above process but instead substitute a temporary mobile broadband service between the landline being ceased after the port away of the number to a VOIP provider, and the same copper cabling then being reused with a new DSL broadband service ordered with a new contract.
The second option would avoid the extra installation costsk,and a property having a second copper line installed only for both lines to be used for a short crossover period. It might well involve the expense of purchasing a 4G/5G mobile broadband router in order to keep the household connected, obviously alongside a mobile broadband plan.