This page presents illustrative flows for the UE Reachability path.
Prior to this flow, an SMS delivery via IMS failed with a particular reason, and the IP-S-MGW has subscribed to receive UE Reachability for IP notifications. This path is where the HSS sends a Sh Push Notification Request indicating that the UE has attached to Packet Radio. The specific cases where the IP-SM-GW requests a UE Reachability for IP notification is described Requesting a UE Reachability for IP Notification.
For details related to the features involved refer to the Fetch IMSI and Ready For SM features.
Triggered by a Push Notification Request (PNR) from the HSS indicating the UE has attached to Packet Radio. The IPSMGW retrieves the associated IMSI and informs the HLR that the UE is Ready for SM delivery.