Why do my iPhone messages fail to send?

Another common reason for a text message failure is a faulty network. When a phone isn't connected to the internet, a slow connection will cause delays in sending messages. In such a case, you should contact your mobile service provider and see if they can resolve the problem. If that doesn't solve the problem, you can try other solutions, such as a new phone or a software update.

See also - https://emailspedia.com/iphone-is-not-sending-texts-to-android/