I have several Cisco 7975s connected to a FreePBX 12 (Asterisk 1.8) server. They work perfectly, but you need to apply a patch to Asterisk.
https://issues.asterisk.org/jira/browse/ASTERISK-13145
Actually, I don’t understand why the perfectly working patch isn’t implemented into Asterisk.
Here is another interesting source:
http://docs.acsdata.co.nz/asterisk-cisco/document-overview.shtml
And it is simply not true that the newest Cisco firmware does not work. I have it installed on all machines…9.4.2.1…no issues at all 
Ciao
Reinhard