I use an 800 number from kall8 for doing just that when I'm traveling, people call us on the toll free number and it forwards out.

It's something like $2 a month, and 7 cents a minute, plus another 40 cents a minute to send it to UK mobile numbers, so for somebody to call and reach her would cost you like 47 cents per minute (much cheaper if it was not a cell phone in Europe)