You will receive a prompt during setup to make and receive phone calls. (If you do not get this prompt automatically, check the notification menu for Phone Setup.) To access from your watch, go directly to the Phone app for setup and follow the instructions to enable phone functionality. In the event you do not receive a prompt, go to Settings in your Phone app on your watch and access the Troubleshooting menu for additional support.
Once enabled, you can use your watch directly to make or receive phone calls. Note that this functionality will consume battery at a faster rate.