Digital Voice is the name BT gives to its own proprietary IP telephony service, it’s unfortunate in my opinion that some competitors use the same ‘name’ for their telephony offers , much like the old saying all Hoovers are vacuum cleaners but not all vacuum cleaners are Hoovers , others taking BT brand name DV and using it for similar products can cause confusion if DV simply becomes an alternative term for VoIP .
BT DV uses a standard UK telephone socket built into the BT router , other ISP’s can choose to offer telephony or not offer telephony ( most don’t ) , thise that do offer a bundled telephone service can choose if that telephony will be delivered via their router ( if it has a phone port ) , or provide telephony much like a third party VoIP supplier does , using an ATA or by providing a telephone with a built in ATA , but these questions are for the individual ISP rather than asking here ( afterall this is a just BT customer forum )
FWIW , the bigger ISP that offer telephony generally are similar to BT in that their IP telephony is designed to use a dedicated phone port on the router they supply, the smaller ones don’t offer a bundled telephone service, but with every ISP , the customer needs to find this out themselves before joining then , if telephony is important to them .
