Disabling and re-enabling iMessages can also fix text messages not being sent
iMessage is Apple's proprietory messaging service that allows you to send and receive messages other iPhone users. It has more features than standard text messages and uses WiFi or mobile data.
In the Messages app 
, you can send text messages in two different ways:
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Over WiFi or cellular service, using iMessage with others who also use iMessage on an iPhone, iPad, iPod touch, or Mac. Your iMessage texts appear in blue bubbles.
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With Text/SMS messages, your Text/SMS messages appear in green bubbles.
Apple ID
You need to have an account with Apple (Apple ID) in order to send iMessages. This is the reason the messages app asks you to sign in with an Apple ID.
Note: Signing in with an Apple ID is not compulsory for diocesan iPhones and iPads. The messages app can function for just Text/SMS messages without signing in.
Warning: To protect diocesan data, do not enter a personal Apple ID (using a non-diocesan email address). Instead request a diocesan work Apple ID by raising a support ticket.
iMessage Tutorial
Note: Diocesan mobile plans include unlimited text/SMS messages and, for Three Mobile users, unlimited data for iMessages. MMS messages (text messages with photos and other media) are chargeable and may be blocked. Alternatively use a share link in OneDrive / SharePoint or send by email.