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Remove Exclamation Mark on iPhone Messages

Open any existing conversation or thread

One opening the conversation the above a error message will pop up

Tap on the Settings from the error message displayed

Turn ON the iMessage by tapping on the toggle button

Once turning ON the status will be updated as "waiting for activation...."

Wait for few minutes, and the iMessage will get activated

Now the exclamation mark on Messages of iPhone will be successfully cleared.