Could the extension 9212 be in use by another PBX in your organization (not CallManager)? That might explain why an external call from the PSTN cannot get to Unity, but internal calls and calls to Unity's pilot (backdoor) do.
What version of Unity, TSP, and Call Manager are you using? Is there anything in the app event log on the Unity box when you try calling that user?
What about the calling search space on that phone? In particular, make sure the Call Forward Busy and Call Forward No Answer extensions on the line point to the voice mail pilot number. Just as important, check that the calling search space on the Call Forward settings allow that phone to actually forward calls to Unity.
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.