I've upgrade my Cisco IP phone 7940 and 7960 model to SIP and use with my asterisk server. I'll describe it here on how to do.
1. You must have TFTP server software. I am using Cisco TFTP. Or you can use linux server to host your Cisco config file.
2. Of course Cisco IP Phone OS (ie : P0S3-05-0-00.sbn)
3. Check your TFTP directory and put 3 important file
a. Cisco IP Phone OS
b. .CNF (ie SIP000628F0FCB5.CNF)
c. OS79XX.txt (Containing P0S3-05-0-00)
Then we move to Cisco IP Phone
1. You must unlock your Cisco IP phone by pressing **#
2. Go to Network and set DHCP to No
3. Go to TFTP server setting and set your TFTP server IP.
4. Everything must be save
5. Restart your Phone
Hope thats can help you