These parameters can be changed, but most of these parameters need to be changed from your Email server (Exchange, Notes, etc.) similar to the policy that your Email team has for normal emails.
However, there are a couple things you can do from the Unity side.
First, under Class of Service --> Messages, there is a check box for 'Deleted Messages are copied to the deleted items folder'. This will allow the voicemail user the option of listening and retreiving messages that the user deleted. However, once again, how long they stay in the deleted folder is dependent on the email server configuration.
Second, there is a tool under the Administration Tools of the Unity Tools Depot called 'Message Store Manager'. This will allow you to control the settings you mentioned above as well as quite a few other options. You can check it out and see if it works for your organization, but we decided to leave the configuration of these settings for our email team.
Here is a little more to add to the great info from Brent! 5 points for this nice answer Brent.
Have a look at these notes :)
Deleted messages can be recovered or retrieved from the Deleted Items folder of the Cisco Unity inbox. This is useful for when the subscriber deletes the message by mistake. But this can also cause problems if deleted messages get accumulated. They need to be purged at some time interval from the Deleted Items folder. Therefore, the Cisco Unity Administrator needs to set a time period during which you can recover deleted messages.
From this excellent doc;
Unity: Set a Recovery Period for Deleted Voicemail Configuration Example
These are the paths to get to each CCX logs through CLI. They may be helpful if you are having issues accessing RTMT or downloading logs through it.
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