To make sure this is all understood:
A user gets a voicemail at 4PM.
If they check their voicemail at 4:45, right before the leave for the day, there are no voicemails.
They show up at 8AM and check their voicemail, they now have a voicemail form 4PM the prior day?
Are they checking the voicemail by dialing in or is the phone alerting them to the voicemail?