If you have telephony from BT and broadband from another supplier ( both sharing the same Openreach copper pair ) then that’s a vary rare legacy arrangement and DV cannot be supplied over it , so to all intents and purposes your ‘date’ will be Jan 2027 , but that won’t be an ordinary migration to DV , before then you will be faced with a choice , join BT for broadband , join the broadband provider for telephony ( if they provide IP telephony ), drop the telephone element and remain with the broadband provider and make your own arrangements with a third party VoIP provider, or join a completely separate provider for one or both services.
If you have service from two providers that requires SMPF ( shared metallic path facility ) and that cannot provide DV , because DV requires BT broadband , as a SMPF customer you are at the back of the migration queue, so you have plenty of time to chose who will provide your services , before you are required to make a choice .
