Voice mail messages are set to expire after 30 days at the time they are first played. Unplayed messages will expire after 90 days.
Note for subscribers who forward their voice messages to email: Depending on the volume of messages you receive, your mailbox could eventually reach full capacity and stop taking messages. Log in to UTVM periodically (perhaps monthly or weekly depending on your usage; disk quota is about 80 minutes of messages) and delete messages.
