You will probably need to rent a gsm phone. (Just search google, lots of locations.) Europe is on a different standard than the US so unless you have a gsm phone (which Sprint is not), it won't work. If you really want to make sure you have service EVERY where, rent a satellite phone. You can get them for under $50 a week and less than $2/min.
(Ok, disclaimer - I use to work in the cell phone and satellite phone industry)