Hi Pete,
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.
Maybe the others can give their advices.
:)