i take it you use wifi of some sort or a network or USB for you internet
if not then you are using dial-up if this is the case you can not send a fax at the same time as being conected to the internet needs the same modem
you need the modem in your pc to be pluged in to a phone point (internet in to part of the system)
the other way you can send a fax is to use a 3rd party host on the internet that will let you send a fax as an email and they forward faxs sent to you to your email this way of doing it dose need the internet
My dial up line is a split line and I can use my telephone while still being connected to the internet. My modem is connected to my dial up line as well as to my computer and there is another phone line from my computer to the phone connection.