While written against Unity 4.x, the Audiotext Applications in Unity paper on the Documents page of my site can still be helpful to you I think - you can find it here:

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

Short version is you either assign an extension to the call handler and let the "attempt forward to greeting" routing rule take you to that handler directly or manually configure a routing rule to direct calls to that call handler based on the dialed/calling/forwarding number(s) of the call. Either way works but if you're simply forwarding from a known extension into Unity and want to fire up a call handler, the easiest way to do that is to assign an extension to the call handler directly and let the default routing do its thing.