to send a fax, but you'll have to connect it separately to the analogue side of your phone line - by plugging it into the telephone socket on the broadband microfilter.
Before you do this, as word of warning:-
If you leave your old modem connected to the phone line you are running a risk. If you somehow managed to download a dialer (a piece of code that can sneak onto your computer without you realising it) you could end up with a massive phone bill. Dialers work by getting analogue modems to secretly dial premium rate numbers and stay connected for hours at a time. The first you may know about it is when a huge bill drops through the letterbox. The chances of this happening aren't great, especially if you practise safe surfing habits and stay away from suspect sites, but the possibility exists.
If you want to send a fax via the analogue modem by all means do so, but unplug the modem from the phone line when you've finished.