1. Login to the Cisco Unified CallManager Administration page with the Username CCMAdministrator (unless you've customized it).
2. Choose System and click Enterprise Parameters.
3. Change the Auto Registration Phone Protocol from SCCP to SIP.
4. Click Save at the bottom of the Enterprise Parameters Configuration page and restart all services in order for the parameter change to take effect.
Note: If you have IP Phones with SIP Auto-Registration enabled and the auto-registration fails, check if the IP Phones still run an older version of SCCP. In this case, SCCP needs to be upgraded to the latest version and then migrated to SIP. But if you try to upgrade the SCCP phone load first, this fails because the phones are set to auto-register as SIP. So, register the phones as SCCP first to upgrade the SCCP load, then change auto-registration back to SIP, restart the Cisco Unified Communications Manager process, and reboot the phones so that the phones are migrated to SIP.
More Information here.