I looked at Tracfone. I could swear that they used to expire your minutes. It looks like they don't do that now, if they ever did. However, they do charge double for roaming minutes.

Actually, doing some research, a lot of reviews claim that Virgin Mobile's minutes expire, which is 100% untrue (well, unless you let your whole phone number expire), so it's possible I got that expiration information on Tracfone from a similar bogus review.