CUCM 11.5 doesn't support 7912 IP Phones any more.
I uploaded sip firmware and set SIP settings at phone web interface (ip_addr/dev)
PWD ***** (digest user password)
LoginID adilm (digest user)
But have no luck. What can be wrong?
That is definitely not supported, and not going to work, the documentation clearly explains that device model will no longer be supported on 11.5 so that you can plan ahead of time and replace them.
Give this a read
Worth checking CUCM logs and Wireshark from phone.