You bet - use the 3rd party fax configuration tool. It ships with later version of Unity (4.x) but can be downloaded from here:

http://www.ciscounitytools.com/App_ThirdPartyFaxConfig.htm

This tool also lets you define which file types Unity will pass through to the fax server when forwarding an email as a fax which can be handy.

There's a help file and training video out there that should get you from A to B on how to use it.