I have uploaded the logs to 3cx and created a support ticket. Below is the updated response I received from 3cx. So will be waiting for the next SP which sounds like it's right around the corner.
Hello,
After looking into the files you provided us with, we found that the specific VMail that had been left ran into a known issue that exists in the current service pack of V15 (SP5).
While we cannot do something actively about this now, I am happy to say that in the upcoming service pack which is currently available in Beta, this has been addressed, so once you upgrade after it has been officially released, you should not have to worry about this happening again.
Please let us know should you have any further questions or inquiries.
Additional Ticket Information:
Ticket ID: 140903
Ticket Subject: Voicemails getting delayed
Ticket Owner: Nikos Tsironis
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Best regards,
Nick Digalakis
Team Leader (Support)