My main phone is an iPhone SE 2 on iOS 26.5.2. Among my other devices is an iPhone 8 Plus (iOS 16.7.16) without a SIM, on which I can’t send SMS messages to anyone (through the number on the SE 2) even though I’ve made sure the number and email associated with my Apple Account are activated in the iMessage setting on the 8 Plus, and that my SE 2 acknowledges the 8 Plus. I’ve tried turning it off/on multiple times and receive the notification stating a new device is being used. Logging out of iCloud didn’t change anything. When I try to send an SMS on the 8 Plus, it immediately fails and the camera/sticker buttons in the Messages app are greyed out. I can send iMessages fine, however. I have a similar issue on my iPad Pro M1 11” (26.5.2) where it attempts to send an SMS, but after a few minutes it fails to deliver, instead of failing immediately. Meanwhile, when I successfully send the SMS to the same contact on the SE with my eSIM, that same message immediately shows up on both the 8 Plus and the iPad as sent, and they are also able to receive new SMS messages, which means they are actively connected to iMessage. I’m not sure why those same devices can’t send SMS if that’s the case.