Probably Skype. As I understand it, the encryptation's decent, and the Iranians I've spoken with seem to trust it as well. We've had pretty good luck connecting into Iran, to my surprise. Bandwidth seems to handle it.
keep it normal you need to think of their safety, right? agree call times. their mornings are best, as UK/USA are offline for some hours. hard as this may sound, but your friends likely need some cover and non-explicit support. make normal calls over regular voice services and keep the conversations off the green wave. tell them you are hearing very little from the media, but you hope everything is fine for them and that they are in good health. discuss their immediate family and friends, without using names. if you've called before, you should know the score. how is mom, niece, etc? i'll call you from my other phone line tomorrow at your 9AM, i'll save money for my next trip to your beautiful country. after all, we are not in a position to actively participate as part of a cell or any organisation inside. if you need to get them information, then TOR/etc. as this site recommends, as information out is more critical to their cause than information in. otherwise, if you really want to discuss anything wet and sweaty, skype is best, while it (or internet) is not blocked.
