What would concern me about that installation is the proximity of the transformer to the phone itself. Depending on how sensitive the phone is to RF you could have a serious humming noise all over your phone lines.
Wouldn't it be easier to just locate the transformer remotely and run the 9 or 12 volt cables to the phone wall plate?
All you would need is a variety of connector options on the plate as many cordless phones and answering machines don't get fed via a pair on the phone jack but rather use a normal round DC connector.
I also wouldn't particuarly like the idea of a transformer burried in the wall where it might be covered up by insulation. Far safer just plugged in to an outlet.
Can't see that device working in Europe with 230V 50Hz systems. It's not usually legal to run phone and power cables in the same box.
