Caller Display Standard
 

I recently discovered that caller display had started to work, yippee! However, it only works on our DECT (wireless) handsets and NOT our old BT caller display corded phone.
I do know the corded phone does work as I have taken it to my in-laws who have BT caller display and it works perfectly there, but fails to display anything on our VM land line.

Can I assume, then, that VM caller display uses a different version of caller display?