The sales people are completely ignorant. This is a digital (broadband) modem. Fax requires an analogue (telephone) modem.
Your broadband splitter separates your phone line into two "streams" - analogue for voice/fax (i.e. normal telephone use) and digital for broadband use. Your USB720 is plugged into the digital stream so there's no way it could send a fax in the conventional way. Your options are either 1. buy a normal analogue modem (which you can use simultaneously with the broadband modem) or 2. use (e.g.) an email-to-fax gateway. That's normally a paid service - see Google for more info.
If you want to send and receive a lot of faxes, you might think about buying a dedicated fax machine.
The moral of this story is never listen to sales people. Always come here first.
