It partly depends on who the telecom service provider is for the public phone booth you are using.
Here is the list of IDD's for France (International Direct Dial code):
00 (France Telecom)
40 (TELE 2),
50 (OMNICOM)
70 (LE 7 CEGETEL),
90 (9 TELECOM
Follow the IDD code with the USA country code: 1
Then, 866-xxx-xxxx
Bearing in mind, that some toll free numbers like 1-866 are only accessible directly from within the USA or Canada.
Also, bearing in mind that, even if the call does connect, I'm not sure the call from a phone booth in France "to" the 1-866 USA number will be free.
If you need an alternative option from France, try CallCentric. They are a VoIP service that provides world-wide calling card gateways for making long distance calls (from regular phones, cells, or phone booth), at VoIP rates, using local access numbers. You can learn more here:
http://www.cqvoip.com
They have two gateway numbers in France:
Marseille: 33-4-86-87-01-82
Paris: 33-1-70-61-54-41