Follow these steps for a good Bluetooth calling experience: 

Android device:

• To mute an incoming call on the watch and phone, press the side button of the watch.

Note: You will also return to the previous level.
• To exit an incoming call from the watch and answer it via your phone, press the side button of the watch.
 also make sure that your phone is connect to calling Bluetooth to use call feature 

iOS device: 

1. When there is an incoming call, press the side button of the watch, no response <remove this>

• To hang up a watch dialed number, press the side button of the watch.

Note: You’ll also return to the previous level in the watch.

• To exit an incoming call from the watch and answer it via your phone, press the side button of the watch.

also make sure that your phone is connect to calling Bluetooth to use call feature