You need to create an online application that is capable of sending a FAX securely (PCI, HIPAA or other compliance).
After trying trustfax and eFax, neither of which has a secure API, Ralph found that Metro Fax has a SSL API for developers and the cost is reasonable.
The following SDK as well as some supporting documentation below will help you get started: WsfSDK
The MetroFax webservice gateway is available at:
And there is supporting documentation (NDoc) available below:
The attached SDK contains sample implementations of numerous common methods.