Dowload Skype here. You can use Skype for chatting, screen sharing, voice calls to other Skype users, conferencing calling. You can even use it to call landline phone numbers!

All participants dial this phone number: 1-605-715-4900
Then, enter PIN: 783014
More information is at this link.
Note: Each call in participant get charged their normal long distance rate, usually .03-.10 per minute.