This is almost certainly a hardware problem with the Hookswitch. After deploying Cisco IP Phones for the last few years we (and many others) have found this to be the most common hardware problem. There is a "Best Practice" for cleaning the Hookswitch contacts (link below) that does work . Usually the phone has to be replaced/RMA'd.
If you don't have smartnet on the phone and you want to buy the parts to fix this let me know and I'll send a link
BestPractices for Cleaning and Troubleshooting Cisco IP Phone Hookswitch
The hookswitch contacts design uses a wipe action to self-clean the contacts. Periods of nonuse of the phone allow air impurities such as dust and other contaminants to hinder contact performance, which results in intermittent operation. Press and release the hookswitch rapidly a dozen times or so to clean the contacts.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.