While you’re on a call
If you’re on a call that doesn’t use FaceTime audio, you can switch a call to your iPhone, adjust the call volume, enter numbers using the keypad, and switch the call to another audio device.
Switch a call from your Apple Watch to your iPhone: While talking on your Apple Watch, unlock your iPhone, then tap the green button or bar at the top of the screen.
You can quickly silence an incoming call by pressing the palm of your hand on the watch display for three seconds. Just make sure you have Cover to Mute turned on—open the Settings app
, then choose a device.
During a FaceTime Audio call, you can adjust the volume, mute the call by tapping 
on your Apple Watch, then tap Voicemail.
On the voicemail screen you have these options:
Adjust volume with the Digital Crown
Start and stop playback
Skip ahead or back five seconds
Call back
Delete the voicemail