Bizarre behaviour of BT 1000 caller display unit

I recently bought a BT 1000 caller display unit. It works fine when
plugged into the phone socket by itself but won't work when a phone is
also plugged in. The BT engineers have been out twice and checked the
line and everything - they are mystified. The problem is not with the
phone as changing the phone doesn't help.

Even odder - if I take phone plus caller display unit to another house
everything works OK.

Any ideas?

Henri

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