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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.