There isn't a COS option for that.
The COS options is "handset volume saved".
When the user sets it they must back it down one notch or it will not save. This was designed with that intent.
Now, assuming that the user is setting it correctly, if the phone is actually loosing it's volume setting then I would check to see if the system is rebooting every night. Even if it was it should still save.
You may have to prove that what the customer is telling you is correct.
Is it happening with every phone?
Is is possible that someone is using that phone over night?
Ralph