When someone chooses to escape our Apple overlords and convert from an iPhone to an Android or other device, they can find that their text messages are no longer reaching them. Apple has confirmed that this issue is caused by a glitch in iMessage.
Apple links phone numbers and text messages with their iMessage system. That means if you have an iPhone and your number is 867-5309, Apple thinks all texts from other iPhones to 867-5309 are iMessages and vice versa. But say Jenny decides to get herself a Galaxy, and keeps her old phone number. All texts from iPhones to 867-5309 should be sent as text messages, but Apple continues to treat them as iMessages, which Jenny can't receive on her Galaxy. Instead of automatically sending the messages as text messages to the newly converted Android user, the texts get trapped in Apple's cloud. Their system has major issues processing a user who converts (perhaps a reflection of their ego.)
Apple has been working on a fix through their servers, but a recent glitch in those servers made the iMessaging issue even worse. Apple's servers intercept the messages before they are sent, determine if the number is linked to an iOS account, then route it accordingly. In order for the servers to intercept properly, they must know which numbers are associated with iPhones and which aren't. Herein lies the issue Apple just can't seem to get around.