How do I stop Apple Watch from ringing when iPhone gets a call?

On your apple watch press the side button (button below the Digital Crown/knob) and depending on the layout look for the gray bell icon and click it to make it red and set to vibrate mode.

If clicking the side button brings up apps, go to the main menu with your wallpaper and swipe from the bottom to top of the screen.

smithurmann wrote:

This suggestion from Grok3 worked.

Open the Watch app on your iPhone. Scroll to Phone under the My Watch section.

Tap Custom. / Under Alerts and Ringtone, turn off the toggles for Sound and Haptic.

This will prevent your Apple Watch from ringing or vibrating, though the call screen may still appear when you raise your wrist or interact with the watch.

That's pretty much what using the Control Center to turn off the sounds does. Except that using the Control Center is faster.

You can either stop mirroring your iPhone settings or on your iPhone settings delete the device connected to your iPhone (Iwatch).