I used Skype a lot while I was living abroad, and it was a lifesaver.
The free Skype requires both people to be on a computer (or, in some cases, a smartphone, although this depends a lot on wireless coverage and spends a lot of data). With this, you can instant message, talk, or talk-with-video. Talk with video is particularly lovely when you haven't seen people in a while.
However, this requires both parties to be on the computer and to have Skype-capable machinery.
I sprung for the unlimited phone calls plus voicemail option, which I didn't regret in the least. With this, you can call any phone in America, cell or landline, for free. You have to be on the computer/smartphone, but they will receive the call on their cell. Even more usefully, people in American can call you on an American phone number (you pick the area code, Skype provides a phone number) and talk to you while you're logged on to Skype. With the voicemail feature, they can also leave you a digital voicemail that you can pick up when you return.
If you're calling a few people a lot, I'd recommend checking out their own mobile plans, as some do charge for incoming phone calls from other area codes. (Which was a pretty big surprise to my then-boyfriend, now-husband, after our first month of communication.)
You call the username. So you will need everyone to get a username.
