If extensions are created using the bulk generator (and possibly individual extensions, but I haven't tested that specifically but do not think it is affected) and the default voicemail settings are configured to attach voicemails to emails, the bulk generation tool adds all of the extensions into thirdlane with the checkbox "checked" for attach voicemails to emails, but it does not properly load the settings into dialplan.
The end result is that it appears that the setting is enabled in the GUI, but voicemails do not get attached to emails.
To fix the issue, you need to edit each extension individually and uncheck the attach voicemails to emails option, save the new settings, go back into the extension settings, and reenable the option. With no bulk update option for this, this is a real pain in the neck that I just had to deal with after importing 47 extensions and needing to manually modify them all.
I'd like to look at this. When i create and update extensions on my test system I do not see any difference in the relevant data right after creation and after the update . Would you be able to give me access to the system where this is happening after creating one "non-working" and one "fixed" extension?