This is a parameter that cannot be changed. It is due to the fact that a 7912, as a single line phone does not have the capability to do anything other than Light the MW Lamp. Only multiline phones like the 7940/7960 etc. have the "Light and Prompt" or "Light Only" "Prompt Only" capabilities.
You do not need to set this. It is the default behavior for a 7912. If using Cisco Unity or Cisco Unity Connection, make sure that you have properly configured MWI On/Off numbers and that the voicemail platform is has ports available and the proper configuration.
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.