If you’re experiencing message send failures, you’ll need to update your iPhone to the latest iOS software.
You can do this by going to Settings/General/Software Update and installing the latest version of iOS available for your device.
If you have an older iPhone (e.g. an iPhone 8 or older) you may not have an update available. In this case, a temporary solution is to turn VoLTE off. You can do this by going to Settings/Mobile/Mobile Data Options/Voice & Data and switching VoLTE off.
We’re currently working on a permanent fix for this issue so that you can enable VoLTE as soon as possible.