Honestly, I do not remember the last time I sent a fax from my Mac. These days it’s scan and email. From MacFixIt:
Besides just scanning and emailing a document to someone, there are a variety of online faxing services that will take a document and send it to a standard fax machine. Some of these are free, and others require a small fee for extra services or large faxes, but should be able to take documents in various formats and relay them to digital or analog fax machines.
Alternatives to using your Mac to fax (and avoiding the need to buy an external USB modem):