We have a situation wherein an iPhone 11 (running iOS 15.4.1) is LTE-attached and registered for VoLTE services but not for SMS over IP (no +g.3gpp.smsip tag in SIP-Register sent by iPhone). In such a case, the iPhone is unable to send user-generated SMS over the NAS interface to the MME. When an SMS is attempted the screen immediately shows a "Not Delivered" message next to the text message. This same issue has been observed with iPhone11 (iOS 15.1) and iPhone XS (iOS 15.4.1).
This issue does not occur if we use an iPhone 12 (running iOS 15.4 or iOS 15.0). With an iPhone 12, the SMS gets delivered correctly to the MME over the NAS interface.
Does anyone know of any issues with iPhone 11 or older iPhone models that causes this behavior please?