It’s open to debate if being able to eavesdrop on a call is a useful function, picking up a handset and being able to listen to someone else’s conversation I’d argue is something to design out rather than design in to the phone’s functionality.
You could get this three way call anyway , if you have multi call, while on the call , set up a three way call , with your own number as the new party to join the call , your second DV phone rings when you make this third party invitation call , then both DV specific handsets can be on the same three way call , that way it’s an invitation to join , rather than someone potentially spying on someone else.
