having a toggle to silence an iPhone's ringtone

That's not a great idea. Imagine, user wouldn't hear the ring phone, without muting intentionnally ?

Not even sure it would pass reviews.

Have a look at those discussions:

https://stackoverflow.com/questions/50737943/how-to-change-volume-programmatically-on-ios-11-4

and the strong warning NOT TO DO:

https://forums.developer.apple.com/thread/104078

Is it possible to create a toggle switch to silence an iPhone's ringtone in my app using swift?

No. If this were possible then it would allow for a discrepancy between the current mute behaviour and the state of the physical mute switch, which would be bad.

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