Okay, you have to get a service provider to allow email to fax, the iburst modem does not use a telephone line to connect to the internet, meaning, that you cannot dial a physical fax number from the iburst modem TO connect to a fax and send it (hence the trouble you're experiencing with Winfax pro and setting things up)
Normal modems that connect via Telkom's infrastructure (aka, modems using normal phonelines) are capable of this.
Next best option is Email to Fax and Fax to Email.