The simpler solution (one that allows the full functionality of the phone with having to patch Asterisk) is to use the chan-sccp-b SCCP channel driver and SCCP Manager packages from Github. My documentation on implementing these is at Interesting new success with Chan-SCCP-B and SCCP-Manager (both from GitHub)
The 7960 phones are effectively crippled using the SIP Load - they are intended to bridge you over to something else that Cisco can sell you. Using the Chan-SCCP-B driver leaves them in Skinny mode and allows the full functionality of the phone to be used. Note that you can install these two packages even if you don’t have any Skinny phones and then set the phone itself up to connect either to SIP or to SCCP.
Remember, if you use the SIP load, you password is an horribly and easily hackable 8 characters, so you SIP port, if exposed to the Internet, can allow for unauthorized access to your system.