I use skype, call quality is pretty good and of course its free when both (or multiple) parties are skyping from a pc. You can also use it to call landline phones and mobiles using skype Out (where you buy credit), and you can recieve calls from landline and mobiles using skype In (you have to pay for this service too - you get a phone number with this service). There is a voice mail service that is free with skype In I believe, otherwise you have to pay for it. I used it to keep in contact with a friend who was travelling last year, I called his hotels to keep in touch when he couldnt get Internet access for skyping on his mini-note and the calls were fairly cheap iirc. My only criticism of skype is that it gets more bloaty with each update; I only use the call service and text chat, its just a shame they dont do a light version.