AT&T MERLIN MAIL System Programming And Maintenance Manual page 191

Troubleshooting
PROBLEM:
Messages cannot be left in a subscriber's voice mailbox.
*-
Possible Cause
The voice mailbox is full.
Voice mailbox may be too small.
6-4
Troubleshooting
Corrective Action
Old messages must be deleted
regularly from voice mailboxes.
NOTE:
When an individual's voice mailbox is
full, new messages to that subscriber
are deposited in the General Mailbox
along with a recording identifying the
name and extension of the called
party.
Mailbox sizes range from 5 to 60
minutes. If subscribers frequently hear
the "Mailbox Nearly Full" message,
you may want to change the Class of
Service to increase the storage size of
their voice mailboxes.