There is a symbol of a bell with a circle and a back slash through it
Next to a name in my messages
iPhone 11, iOS 15
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That means that the person is muted so that if they text you your phone won't go off. Helpful if they are being rly annoying but still want to hear other people who are trying to reach you. Simply swipe to the left on the bar of the person in messages and tap the purple square with the bell to unmute them.