I would like to begin by stating that it is NOT a known problem with the Apple Watch.
Your series 9 Apple Watch GPS+Cellular can make and receive calls when it is connected to a cellular network. How are you checking to ensure that the Apple Watch is connected to LTE (Cellular) network? When you turn your iPhone OFF, then the Apple Watch first connects to a known Wi-Fi network and then if Wi-Fi is not available, it connects to the cellular network.
When you try again to make a call with your Apple Watch, ensure that it is connected to the cellular network. You can check this by pressing the SIDE button and looking in the Control Center. The Cellular icon should be GREEN. If the cellular icon is highlighted in WHITE, then the Apple Watch is not connected to the cellular network.
If the above does not work, then you will have to ➡️ Unpair and erase your Apple Watch (Keep your Cellular plan on the Apple Watch when prompted. Do not erase the cellular plan) and then you will have to ➡️ Set up and pair your Apple Watch with iPhone.
You can also choose to skip all of the above and have your Apple Watch examined at the Apple Store 👉 Genius Bar Reservation and Apple Support Options or at an Authorized Service Center for further diagnosis and evaluation.
Axel F.