If you’re dealing with an overseas supplier and saving your phone bill is a consideration, then I’m sure you’ll be familiar with Skype (skype.com) and this can be a big asset (both for calls and quick messaging chats to check on progress, swap files etc).
Otherwise for something a bit more solid, or if a conference call is called for, I’d again recommend gotomeeting.com There are others but I’ve consistently found this works well both through Mac or PC (with microphone /speakers) or by allowing people away from a computer to dial a number to join the chat as well. Screen sharing and all other expected features are included too.
