If you currently pay BT for an answering service (so not an answer machine in your own home ) then DV provides a basic answer service for free , but you need to cancel the old ‘PSTN’ paid for answer service you currently have yourself.
TBH , the DV answer service can be basic and free , or a paid for premium version , you would need to check or perhaps someone will advise if recording a personal greeting as opposed to the standard BT greeting is on the premium or basic DV answer service but if all you want is a way for a caller to leave you a message then the free service is perfectly adequate , and will save you whatever you currently pay for the old answering service ……the BT Duet 500 doesn’t seem to have a built in answer machine so there shouldn’t be any conflict between your equipment and the DV answer service , sometimes a customers own answer machine and BT 1571 answer service ‘compete’ to to record any message left , sometimes the customer isn’t aware where the message was left (so for example check their own machine but not the BT answer service
