From the main menu, select Setup > Messages.

Ring Until Read

Sets the device to ring until you read a new message. This feature is helpful if you are in a noisy environment.

Adaptive Mail Check

When this setting is enabled, the device listens more frequently for a reply to a sent message for ten minutes. If the recipient replies quickly, you can receive the reply before the next scheduled listen interval. This setting is always enabled on consumer and professional flex accounts. On byte-based plans, it can be disabled to reduce battery consumption.