I am having the almost the same issue. If extension 20 is called it forwards to voicemail properly. If my ring group forwards to the voicemail for 20 it does not play a greeting and will not allow you to leave a message, however extension 20 lights up says it has a message and when you check the message there is no recording…
There are three different voicemail types.
If you set the ring group’s “fail” destination to the “No prompt” voicemail destination, it would do precisely what you are describing.
Everything functions like it should, it only doesn’t work when the call comes from the outside trunk.
Well, there you go. Problems solved then.
No, I still have the original issue. Voicemail functions properly from internal calls, but does not from external calls through a SIP trunk.
Sorry, I thought you meant that you just wanted it to magically disappear. 
What did you try and what was the outcome? You need to be the one that narrows your problem down. All you’ve said so far is “I have a problem.” This isn’t commercial support, it’s a discussion forum.
Did you check what I asked you to check?
This problem sounds amazingly like a problem people were having with ring-groups a couple of weeks ago. Have you gone back through the discussions here to see if you can find what they did?
Post a sanitized call trace of a failed call to voicemail. My guess is that it’s a codec mismatch.