This entire thread assumes the subscriber is happy to use BTs Digital Voice Service. If they wish to port their number to another VoIP provider what then?
Then they lose their broadband service.
You cannot port out the landline number of a
bundled BT broadband+voice service without it triggering a cease of the broadband.
If someone isn't happy to use BT Digital Voice then they shouldn't take Voice from BT. There's nothing in their contract that states how the voice service is delivered so at any point BT can (and will) switch it to VOIP and there's nothing the customer can do about it.
Almost all new BT FTTP orders are now provisioned on Digital Voice with new BT FTTC orders now going the same way.
Existing BT customers with PSTN are being migrated to VOIP as quick as BT can keep up with Hubs and VOIP handsets.