Starting in the Helsinki release, a new notification subscription model was introduced. In the current subscription model, simply enabling one or more devices for a notification in the user's notification preferences does not subscribe the user to a subscribable notification. This only allows the user to opt in to receive a notification for which they might be included in the Users, Users/Groups in fields, or Groups fields of the notification's Who will receive section.
Helsinki and up.
The user may not have subscribed to the subscribable notification by creating a personal notification.
In the current subscription model, when a notification is subscribable (the Subscribable checkbox in the Who will receive section of the notification is checked), users who want to subscribe to the notification must create a personal notification and select the notification in order to subscribe to it: Create personal notifications
This will add a row to the Notification Subscription [sys_notif_subscription] table, associating the user to the subscribable notification. If no such row exists, the user is not subscribed to the notification under the current subscription model.
After creating the personal notification, users will still need to enable the device or devices of their choice in order to receive notifications on that device. In this example, the user has enabled email but not his or her cell phone: