Any voicemail messages in your Voicemail Mailbox that are saved for 10 days will be automatically deleted. This is to save space for new voicemail messages.
There are few things you can do to keep those voicemail messages.
1. Change your Voicemail Settings to send you a copy to your email
2. Dial in to your Voicemail Mailbox remotely or locally, listen, then save the message for an additional 10 days.