What do sent, delivered, read and failed mean for my WhatsApp API messages?

A WhatsApp API message is sent (one tick), delivered (two grey ticks), read (two blue ticks) or failed. Read only counts after delivered.

• Sent: the message left Meta's servers.
• Delivered: it reached the customer's phone.
• Read: it was shown in an open chat. If the customer opens the chat at once, the delivered update may not arrive.
• Failed: it could not be sent or delivered, and the status comes with an error reason.
• Use failed to call the customer or send the update again.