To send a voice message on an iPhone, follow these steps:

1. Open the Messages app and start a new message or open an existing conversation.
2. Tap the plus "+" button next to the text input field.
3. Select "Audio" from the menu that appears. This will start recording your voice message immediately.
4. Speak your message. When you finish, tap "Stop."
5. You can listen to your recording by tapping the play button, or tap "Send" to deliver the audio message.
6. If you want to cancel and re-record, you can do so before sending.

Recipients can play the audio message anytime after receiving it, but the message will be deleted from your iPhone after 2 minutes unless you tap "Keep" to save it

. This method requires iMessage to be enabled and works between iOS devices. The audio message is sent within the Messages app and can include a transcript if supported in your language settings

. In summary, tap "+" > Audio > record > stop > send to send a voice message on iPhone.