BT Diverse 6250/6210 Phone problem ?

I have a BT6250 Diverse Cordless Phone/Ansaphone as a main phone downstairs with a 6210 unit upstairs.On the main phone callers cannot hear me speaking although I can hear them perfectly. If I use the upstairs phone there is no problem even if I take it downstairs.I have referred to the user manual and checked the "Secrecy" facility which always appears to be "On" when I press the button. Is there a way of turning it off that I don't know of or is there another fault with the unit. I have tried deregistering the unit from it's base and registering it again.