If you are talking about the built-in Unity IP fax capabilities, I can tell you that they don't scale.  The Unity IP Fax feature is

only intended to for organizations that send and receive  a combined total of fewer than 50 faxes a day.  The gateway side depends on T.37 fax capabilities on the gateway.  For outbound, what it is doing is using the T.37 offramp feature in IOS and a TCL script that you install on the gatweay to send faxes outbound.  So, really what the gateway can do and what Unity IP Fax are intended for may be quite different.  Start here:

http://www.ciscounitytools.com/Applications/Unity/IPFaxConfigurationWizard/Help/IPFHelp_ENU.htm#_Toc123614078

It will have all the pertinent links for you to look at and see what the capabilities are.