Can I clarify, from your 7906 phones, you can dial into the VM and hear the greetings. And when you dial from your 7940 phones, can you dial into the VM at all? Or you can dial in and not hear anything.
If it is the first issue, then maybe I would suspect the Voicemail ports CSS not being able to reach the phone partitions.
If it is the second, it could be a codec issue. Just guessing for now.
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.