Issabel is built from packages, not from source. Unless you can persuade the Issabel folks to add the GVSIP-NAF components to their packaged version of Asterisk, your only other option is to bring up a source-based Asterisk server, load GVSIP-NAF onto it, and then link to the GVSIP-NAF host with a SIP trunk from Issabel. See this thread for tips.
I took your advise and installed a new IncrediblePBX on hiFormance. Everything works except AvantFAX doesnt let me login with the default "Admin, password". Do I miss something?
Again, Thank you for sharing.