Hi Sampino1,
Thank you for using Apple Support Communities. I understand you're having difficulties receiving calls on your Apple Watch when your iPhone is powered off. I'd be happy to assist.
I'd recommend taking a look at this resource that talks about using the Apple Watch without an iPhone: Use your Apple Watch without your iPhone nearby
Specifically, take a look at these two notes:
"1. To receive SMS, MMS, or push notifications from third-party apps on your cellular Apple Watch, your paired iPhone must be powered on and connected to Wi-Fi or cellular, but it doesn't need to be nearby. You also need to be signed in to iMessage on your iPhone.
2. If you're on Wi-Fi and want to call a phone number, make sure that Wi-Fi calling is turned on for your Apple Watch. Otherwise, try making a FaceTime audio call."
Wi-Fi and having Wi-Fi calling enabled would be required to make a call from your watch with your iPhone turned off. An alternative route would be to make a FaceTime audio call which would function over cellular data.
Here's how to set up Wi-Fi calling on the Apple Watch: Make a call with Wi-Fi Calling
Thanks and have a wonderful day.