Answer
Before making changes, print out what you have in case you make a mistake

If you want calls to Forward No Answer more than one step from your phone, you add CLS SFA to the place you FDN.

Caller    --> Your Phone --> 1st Place you FDN --> 2nd Place you FDN
Dials You     CLS FNA        CLS FNA SFA           Will ring forever

Link to this page: http://www.ghtrout.net/rdr/sfa.html