SMS Messages coming to Apple Watch but not iPhone.
This issue was only happening with one friend, who had an Android phone but iPad and Apple Laptop. So her email address sent and received iMessages in Blue, but her phone number sent SMS or RCS Messages in Green. I could send Messages to her phone but her replies only came to my watch.
I fixed it by going into Settings/Apps/Messages/Send & Receive and setting 'Start New Conversations From' to my phone number. iPhone 15 Pro, iOS 18.0.1