I've got caller display (had it for some years) which is why the call blocker worked OK before Infinity was installed. I should have said that the phone works fine without the call blocker in the line either directly from the main box or from an extension.
Someone did suggest that the line polarity might have become reversed, but I wouldn't have the faintest idea which wires to swap over! (And I couldn't do it from inside the main box anyway, that's a definite no-no but might be possible from an extension connection in the main box which I am allowed to play with).
GG