Sending a fax with VoIP can work, but is not a feature offered by very many providers at this time. Vonage and SunRocket are currently the only providers that we are aware of who offer faxing services.
Faxing via any Internet phone service provider is still considered to be a challenging transaction. The reason for this is because the fax must be converted into an Internet format, sent out, and translated back on the receiving end. With each advancement in the quality of the fax machines made available, the quality of the VoIP fax will continue to improve as well.