Call Flow Diagrams
This document contains sequence diagrams for all major call scenarios handled by OmniMSC. Each diagram shows the signaling flow between network elements.
For CC FSM state definitions, see Control Panel Guide. For routing configuration, see Routing Configuration. For SIP trunk signaling details, see SIP Trunking. For ISUP trunk signaling, see ISUP Trunking. For media gateway control during call setup, see Media Control. For supplementary service flows (hold, MPTY, ECT), see Supplementary Services.
Mobile Originating (MO) Call
A subscriber initiates an outgoing call. The MSC handles call setup, authentication (if needed), cipher mode, assignment, and trunk allocation.
Mobile Terminating (MT) Call
An incoming call arrives from the PSTN or a SIP peer. The MSC pages the subscriber, sets up the radio path, and connects the call. The trunk-side signaling may be SIP or ISUP depending on the route.
Call Hold and Retrieve
Call Hold and Retrieve allow a subscriber to suspend and resume an active call per 3GPP TS 24.083. While a call is held, the media gateway places the held leg into receive-only mode, silencing the audio path. The subscriber may establish a second call while the first is held.
The HOLD message from the UE contains no parameters. The MSC responds with HOLD ACK on success or HOLD REJECT if the operation is not permitted (for example, if the subscriber does not have Call Hold provisioned). RETRIEVE follows the same pattern with RETRIEVE ACK or RETRIEVE REJECT.
Multi-Party Conference (MPTY)
MPTY allows a subscriber to bridge multiple calls into a conference per 3GPP TS 24.084. The subscriber acts as the conference controller. All conference audio is mixed through a conference bridge on the media gateway.
For configuration, see Advanced Call Features.
BuildMPTY
The subscriber establishes two calls (one active, one held), then invokes BuildMPTY via a CC FACILITY message to merge them into a conference.
HoldMPTY, RetrieveMPTY, SplitMPTY
Once a conference is established, the controller has three additional operations available:
| Operation | Effect |
|---|---|
| HoldMPTY | Places the entire conference on hold. All remote parties hear silence. The controller may establish a private call outside the conference. |
| RetrieveMPTY | Resumes a held conference. All parties are reconnected to the conference bridge. |
| SplitMPTY | Extracts one party from the conference into a separate held call. The remaining parties continue in conference. If only two parties remain after the split, the conference context is released and the call reverts to a normal point-to-point connection. |
Each operation is invoked by the UE via a CC FACILITY message and acknowledged by the MSC with a corresponding return result or error.
DTMF Relay
OmniMSC relays DTMF tones from the radio interface to the trunk side. The UE sends a START DTMF message (3GPP TS 24.008) containing the digit. The CC FSM immediately acknowledges the UE and forwards the digit to the SIP peer.
The SIP INFO body uses the application/dtmf-relay content type with Signal and Duration fields. The MSC sends START DTMF ACK immediately to the UE without waiting for the SIP peer's response, avoiding audio clipping on the radio path. DTMF relay is handled in any call state where a trunk-side connection exists.
Emergency Calls
Emergency calls receive priority handling. The MSC detects emergency calls by the CC Emergency Setup message type (3GPP TS 24.008 §9.3.8, message type 0x0E) and the CM Service Request type (:emergency).
Authentication is attempted normally. If it succeeds, cipher mode is established and the subscriber's MSISDN is used as the calling party. If authentication fails, the call proceeds anyway — cipher mode is skipped and the IMEI is used as the calling party instead. Calls from UEs without a SIM are accepted.
Unlike regular CC Setup, the Emergency Setup message does not carry a Called Party BCD Number IE — the handset sends only an optional Bearer Capability and Emergency Service Category. The MSC uses the configured psap_address as the called number for route table lookup and the outgoing SIP INVITE Request-URI. See Configuration Reference for emergency parameters.
CC FSM State Diagram
The CC FSM (Call Control Finite State Machine) manages the lifecycle of a single circuit-switched call per 3GPP TS 24.008. The MO and MT paths share common states but enter through different transitions. Live call state, duration, codec, and serving BSC/RNC are visible in the Active Calls page of the control panel — see Control Panel Guide.
MO Call States
MT Call States
Connection Release
When the A-interface connection is lost (radio link failure, BSC clear, or transport failure), MSC-A sends a connection_lost event to all CC FSMs associated with that subscriber. Each CC FSM receiving this event releases its trunk-side resources (SIP BYE or ISUP REL) and media gateway connections (MGCP DLCX), then transitions to the null state and writes the CDR.
This ensures that no trunk or media resources are leaked when the radio path is unexpectedly lost. The CC FSM handles connection_lost in any state except null.
Location Update
Subscriber registers with the MSC. The MSC authenticates the subscriber and updates the HLR.
Intra-MSC Handover
Handover of an active call between two BSCs served by the same MSC.
Inter-MSC Handover
Handover of an active call from OmniMSC (MSC-A) to a target MSC (MSC-B).
SRVCC (Single Radio Voice Call Continuity)
Handover of a VoLTE call from the IMS/LTE domain to the CS domain per 3GPP TS 23.216.
CAMEL-Triggered Call (SCP Interaction)
Call with CAMEL Originating BCSM (O-BCSM) interaction per 3GPP TS 23.078.
MPTY (BuildMPTY) Sequence Diagram
Multi-party conference call setup. The subscriber establishes two calls, then bridges them into a conference per 3GPP TS 24.084. For configuration, see Advanced Call Features.
ECT (Explicit Call Transfer) Sequence Diagram
Explicit Call Transfer connects two remote parties and releases the transferring subscriber per 3GPP TS 24.091. For configuration, see Advanced Call Features.
CSFB MT Call Sequence Diagram
Mobile-terminating call to an LTE-attached subscriber via Circuit-Switched Fallback per 3GPP TS 23.272. The MSC pages via the SGs interface, the UE falls back to 2G/3G, and the call proceeds over the A-interface. For SGs details, see SGs Interface & CSFB.
SIP-I Call Sequence Diagram
Outgoing call via SIP-I trunk with encapsulated ISUP per ITU-T Q.1912.5. The INVITE carries a multipart body containing SDP and the ISUP IAM. For SIP-I details, see SIP-I Trunking.
ISUP-to-SIP Transit Call
Transit call interworking between an ISUP trunk and a SIP peer, bypassing the CC FSM.