I have the exact same problem--I can call out, but still cannot receive calls on my new apple watch 8 paired with my iPhone 12 pro max. All software versions are current. I've checked with the Genius Bar and my phone and watch are set up correctly. I've been on the phone with ATT several times now. Tried to reinstall the EID and eSIM. Paired and unpaired the devices multiple times and powered down the devices each time.
While working with ATT, the last time, they were able to call me on my watch while my phone was turned off but then it stopped working immediately when my husband tried to call me. I got disconnected with ATT after this and now I don't know what to do. Every time I call them, I have to start all over... VERY frustrated.