As Lola said, it does not matter where the phone is - and if you want, you can set up an automatic top off schedule to keep your phone active without having to keep track on your own. The credits will simply accumulate until you get back & start using it.
Lola? Your phone is probably more advanced than mine - if its a quad band it will work just about anywhere. However, my little simple one did pick up a signal on a cruise ship out of Miami.
JA - soon come....