Different business scenarios can call for the message alert on your iPhone to be set to different levels. For example, if you're expecting a very important text message and you know you won't be in a situation where the message tone will bother anyone else, you may want the message alert volume set very high to ensure you know when the message comes in. Alternatively, if you know you're going to be in a meeting, but you still want to be made aware when a message alert is received, you can set the tone to a very low volume. It's also important to consider that the ringtone and text tone volumes are tied together on an iPhone.
Tap the "Settings" app on your iPhone.
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Hold your finger against the circle icon on the slider bar in the Ringer and Alerts section.
Move your finger to the left to move the circle icon toward the speaker icon without sound wave icons coming out of it to reduce the volume of the tones associated with incoming phone calls and text messages. Alternatively, move your finger to the right to move the circle icon toward the speaker icon with sound waves coming out of it to increase the volume of the tones associated with incoming phone calls and text messages. A preview of the volume level plays while you're moving the circle icon around.
Remove your finger from the screen after finding a volume level that works for you. The change is applied automatically.
Press the "Home" button to close the Settings app and return to your iPhone's home screen.