It is very nice of you to think of the soldiers who could use your extra phone minutes. Unfortunately, the card you have is probably for domestic minutes, and the amount of time that would be eaten up on making an international call would probably not make it worth while. I also doubt that it would get to a soldier within 10 days, even if you were to mail it today.
I know that my DH uses an AT&T calling card that I recharge for him from home. It uses up about 3 minutes for every domestic minute I put on the card. I still consider myself VERY lucky that I get to talk to him as much as I do. He waits in line for sometimes an hour at a time on the days he's not in the field so he can call home and let me know he's ok. He knows that I worry every day that goes by when I don't hear from him.
Thanks again for thinking of our soldiers. Even though they probably can't use the card, the thought is very much appreciated!