IPhone 7 ringer glitch?

I recently got the iPhone 7 about a week ago and I've lately been having a problem with the ringer. If I use the switch on the iPhone to mute the ringer, the volume goes all the way up and just stays up. The banner that shows you the volume stays on the screen and if I try to lower it, it'll just go back up. I don't think it's because my ringer button is dirty because I just got the phone. What should I do?
Edit

Try a forced reboot to begin with.

Hold both the power + volume-down buttons for ten seconds or so until the apple logo appears. That does not delete anything, it's just equivalent to a hard reset on a PC.

If that does not work, contact Apple support - it's under warranty so their job to fix.

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