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While experimenting with CSFB, it was observed that when the UE returns to E-UTRAN after a CS call, the UE performs a Tracking Area Update with a combined Tracking Area/Location Area update and IMSI attach. Currently, Open5GS's MME simply responds with a TAU Accept message but does not inform the MSC/VLR. As a result, no further MT (Mobile Terminated) CS/SMS services are possible in cases where the MSC/VLR only attempts paging on GERAN. However, some MSC/VLR implementations with fast fallback may still attempt paging on E-UTRAN, allowing MT CS/SMS services to function intermittently. According to 3GPP TS 29.118 Section 5.2.2 Procedures in the MME, specifically Section 5.2.2.2.1, if the timer Ts6-1 is not running, the MME shall start the location update for non-EPS services procedure upon receiving a combined Tracking Area Update Request indicating combined TA/LA updating with IMSI attach. However, SGs timers are not implemented in Open5GS, which is a separate issue. To comply with the specification and ensure that the MSC/VLR is informed when the UE becomes reachable via SGs, the following changes have been implemented: 1. Delay UEContextReleaseCommand: When the active_flag is set to 0, the UEContextReleaseCommand is now delayed until the MME receives the TAU Complete message from the UE. This ensures that the UE has acknowledged the new P-TMSI before the network releases the context, maintaining proper synchronization between the UE and the network. 2. Include Mobile Identity Only When P-TMSI Changes: The Mobile Identity is now included in the Attach/TAU Accept messages only when the MSC/VLR updates the P-TMSI. This ensures that the UE receives the Mobile Identity information solely when there is an actual change in the P-TMSI, preventing unnecessary or incorrect handling of TAU Complete messages. 3. Send SGsAP-REALLOCATION-COMPLETE Conditionally: The SGsAP-REALLOCATION-COMPLETE message is now sent to the MSC/VLR only upon receiving a Attach/TAU Complete message from the UE. This confirmation indicates that the UE has successfully updated its P-TMSI, ensuring that the MSC/VLR is accurately informed of the change. 4. Handle P-TMSI Confirmation: When the MSC/VLR updates the P-TMSI, Open5GS stores the new P-TMSI in the next field of the mme_ue structure. Upon receiving the TAU Complete message from the UE, indicating acknowledgment of the new P-TMSI, Open5GS confirms the update by transferring the P-TMSI from the next field to the current field. This ensures that the MME maintains an accurate and up-to-date record of the P-TMSI as confirmed by the UE.