The best offer is Zen card that you can buy from most kiosks. There is a start-package that costs 2.95 and includes 3 euros worth of call-time. At the moment there is a campaign where buying 1 start-package gives you two additional sim-cards that are not charged, which you can then give to other members of your group. If you charge it with at least 10eur (you buy a charging card worth 10 eur), you can call and text other Zen users for free. If you load 15 eur you can also surf the internet for free (providing you have that kind of a phone). More info from http://www.zen.ee, use google translator.