Looks like this was a bug for some people in prior editions as well, but I have it after updating to the latest public beta this afternoon. I get this error message, even when correctly signed into the same account in FaceTime and iCloud:
FaceTime and iCloud must be signed in to the same Apple ID to use iPhone Cellular Calls.
FaceTime and iCloud must be signed in to the same Apple ID to use iPhone Cellular Calls.