Phone Card - London & Paris to US

Can anyone point me to a phone card for making calls back to the US, with
the calls originating either in London or Paris? Should I buy the card in
the US before I leave, or wait until I arrive in London to get one to get
the best price. Especially need a local or toll free access number that will
work from both London/Paris. Thank you.

I ended up using the Bizon.com card. It's pretty good; the challenge is
getting access to a telephone to use the card! It's great if you're
staying with someone who has regular telephone service as their home. You
get a toll free local number in both London and Paris (and plenty of other
places). BUT, and this is the BUT, if you're in a hotel, you have to pay
through the nose just to use a phone at all, even to access a local number.
Same with phone booths. So, the card was great, but it won't be your only
expense when you go to make a call. -- t