Unless they have an International system cell phone, you won't be able to contact them on that. They probably discovered that too, and are coming home Friday so just decided not to call. Pay phones over there are not easy to use, and it can be really pricey too.
I was in Malaysia and couldn't use my cell phone, pay phones would only work with a phone card, and you couldn't call collect or to a cell phone.
Don't worry. It's probably all right.