Using OS X 10.4.2 to send a long-distance fax using a calling card isn't as easy as I would have hoped, but here's how I did it. (Yes, I tried a dialing prefix, and I tried adding ,pauses, but it didn't work.)
I opened the document to fax, File:Print, click on the PDF button, Fax PDF ... Pick a fax destination (click on the person silhouette, use TAB to navigate). Do what you want for Fax Cover Page, and then change it to Modem in that pop-up menu. Uncheck "Wait for dial tone before dialing" and then reach for the phone. Do all of the dialing (yes, all), and finally click on the Fax button. Yes, it tries to dial again, but that didn't matter.
Looks like I'm not the only one trying to send a fax with a calling card. See Mac OS X Hints for another very similar version.