To stick to your original point , the free BT DV phone ( Digital Voice isn’t a generic term for IP telephony but the name of BT’s proprietary IP telephony offer ) supplied , is so the BT consumer can use the DV service, chances are they could have done that anyway, as the consumers existing regular phones would almost certainly have worked anyway .
Once someone moves away from BT and DV , why would they expect the BT DV equipment to work with a competitor’s equipment , if ( for example ) someone had Sky TV and leaves for Virgin, they don’t expect the Sky set top box to work with Virgin do they ?
If the consumer has to make a separate purchase so they can use a service they are paying for , that presumably should influence their choice of provider in the first place , after all it makes no sense ( for example ) to change provider to save £2 a month on a 24 month deal only to have to spend £50 extra on new equipment, so they they can use the new providers services.
Even if the DV phone was supplied without ‘strings’ , so it was genuinely free , there is no obligation for the supplier of it , to make it compatible with anything other that the product it was supplied with , in fact it would be commercially nonsensical to make it so .
if you are with VF and they don’t supply something necessary to access their version of IP telephony ( hopefully they don’t call it DV as that’s just confusing ) then obviously you are at liberty to use other equipment you already have that is compatible or purchase something new , but its unclear why you would have an issue with BT about your VF service and your presumably inability to use it without spending some more money on new compatible equipment
