My 990 students converted me to this service for conference calling and international calling--all you need is a headphone with a microphone.
It's free!
And you can IM using it too.
http://www.skype.com
Now, when you're planning international calls, it's important to get the time right. I use this site for meeting planning: http://www.timeanddate.com/worldclock/meeting.html.
And to invite people to a meetings, consider using http://diarised.com/ --from Kara Penn