No Cisco phones discussion would be complete without mentioning the SCCP Channel Driver (see Interesting new success with Chan-SCCP-B and SCCP-Manager (both from GitHub)).
The advantage is that it doesn’t require you to patch Asterisk and it works with FreePBX. It’s an easier solution, and if you do it for each of the Asterisk versions you want to use, you could even (theoretically) use the asterisk version switch script to move from version to version.