I found the answer to this question by gut instinct so I'm sharing it here.
For IPhones 10 or X once you slide the button on the side to unsilence the phone, next turn the phone off then back on. The phone is no longer in unsilence mode.
I had DO NOT DISTURB off and followed all other directions but nothing worked until I turned off the phone.
iPhone X, iOS 14
What you're describing is not normal behavior. The mute switch should not require turning the phone off and back on. If your mute switch isn't working properly, you may well have a hardware issue. I'd suggest you contact Apple if you keep having this problem.
Genius Bar Reservation and Apple Support Options - Apple
The mute switch on the side of the phone and the Do Not Disturb feature are not linked.
What you're describing is not normal behavior. The mute switch should not require turning the phone off and back on. If your mute switch isn't working properly, you may well have a hardware issue. I'd suggest you contact Apple if you keep having this problem.
Genius Bar Reservation and Apple Support Options - Apple
The mute switch on the side of the phone and the Do Not Disturb feature are not linked.
Unsilence IPhone X
