What's the difference between inactive device, failed notification, and invalid device during notification delivery?

Question 

When a problem occurs with a notification delivery we sometimes see status failed, device inactive, device invalid... how do those all relate and what does each one mean?

Environment

All versions of xMatters On-Demand

Answer

There are a few nuances between these things, but hopefully the following points should help clear things up.

• A Failed Notification is mentioned when something went wrong in the delivery, which could be any  number of reasons from the provider was unavailable, the message took too long to send, or something downstream went wrong. There's probably nothing wrong with the device details.
• An Inactive device is a device that is set to inactive in the user's profile. It's probably valid syntactically, and could have been turned off deliberately by the user or their supervisor, or automatically because there were too many failed attempts to contact it. To find out how this could happen automatically after repeated failures see the page on Managing devices in the online help (specifically, the Activate or deactivate a device section).
• An Invalid device means there's something wrong with the device entry - this may be a phone number with too many or too few digits, a bad area code, an email address without an @ symbol, an SMS number that cannot possibly exist, etc.