I think it might be Orange.
I'm sure I read that they have a multi-phone deal, with calls between up to four seperate phones all free and one bill for the lot.
We might look at such a deal as our children get older. Currently, only the 13 year old has a mobile, but the other three will soon catch up, and it could be a great way to keep us all in touch with one another.
Let me know what you find out.
Cat