Communication channels settings have Email and SMS notifications enabled by default in Admin panel -> Guest messages -> Texts and Emails tab.
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That means that your guests will receive all relevant email and text notifications (if enabled) about their reservation.
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If you'd like to disable either of those or both, go to Admin panel -> Guest messages -> Texts and Emails tab.
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Important note: You must disable each Notification template separately. This allows for more flexibility in terms of chosen notifications. Some restaurants prefer not to inform guests about changes made to their reservation, for example.
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Uncheck the boxes to appear white as shown below.
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Make sure to click the orange rectangle Save button. It will appear after you make changes to the settings.