Thank you Swiss,
and now I'm going to disagree with you too.
In so far as you gave your friends complete systems, and they used transformers to convert power, I will agree that that may well work (even in the UK), the original question here involved mixing handsets from the UK with handsets from the US. That won't work, regardless of how many transformers you add.
Russ
We kid because we love.
Bang & Olufsen Tysons Galleria
McLean, VA USA