I believe that as long as you have some credit on the phone, the sim will not expire. We last used/topped-up our Digicel phone back in November. When we returned from our trip, I put it in a drawer and forgot about it until I saw this thread. Yesterday, I charged up the phone, checked my balance and found that I still have over $1200J on the phone and I can still make and receive calls - 4 months later. One time, I think I went over 6 months between top-ups and the sim didn't expire as I still had some credit on the phone.