Advanced Call Control Features

This document describes the advanced circuit-switched call control features implemented in OmniMSC, including multi-party calling, call transfer, call completion, call deflection, multicall, priority services, and charging.

For basic supplementary services (call forwarding, barring, CLIP/CLIR, USSD), see Supplementary Services. For call flow diagrams, see Call Flow Diagrams. For configuration parameters, see Configuration Reference. For general operations, see Operations Guide.

---

MPTY (Multi-Party / Conference Calling)

MPTY allows a subscriber to establish a conference call with multiple remote parties per 3GPP TS 24.084. The subscriber acts as the conference controller, bridging held and active calls into a single multi-party conversation.

MPTY Sequence

MPTY Configuration

Parameter	Type	Default	Description
max_mpty_parties	integer	6	Maximum number of parties in a single conference (including the controller). Per 3GPP TS 24.084, the minimum required is 3.
mpty_tone_on_join	boolean	true	Play a notification tone when a party joins the conference.
mpty_tone_on_leave	boolean	true	Play a notification tone when a party leaves the conference.

---

ECT (Explicit Call Transfer)

ECT allows a subscriber to connect two calls together and then drop out of the connection per 3GPP TS 24.091. The subscriber must have one active call and one held call. After ECT, the two remote parties are connected directly through the MSC.

ECT Sequence

ECT Configuration

Parameter	Type	Default	Description
ect_alerting_allowed	boolean	true	Whether ECT is permitted when the second call is in alerting state (not yet answered). When false, both calls must be answered before ECT.

---

CCBS (Call Completion to Busy Subscriber)

CCBS allows a caller to request automatic callback when a busy called subscriber becomes free, per 3GPP TS 24.093 and 3GPP TS 23.135. The MSC monitors the called subscriber's state and initiates a callback when they become idle.

CCBS Flow

CCBS Configuration

Parameter	Type	Default	Description
ccbs_queue_size	integer	5	Maximum number of pending CCBS requests per subscriber. Additional requests are rejected. Per 3GPP TS 23.135 Sec 4.2.
ccbs_supervision_timer	integer	180	Supervision timer in seconds. If the called subscriber does not become free within this period, the CCBS request expires.
ccbs_recall_timer	integer	20	Time in seconds to wait for the originating subscriber to answer the CCBS recall.
ccbs_retain_timer	integer	30	Time in seconds to retain the CCBS request after the called subscriber becomes free, in case the subscriber becomes busy again before the recall completes.

---

Call Deflection

Call Deflection (CD) allows a called subscriber to redirect an incoming call to another number before answering, per 3GPP TS 24.072. Unlike call forwarding, CD is an on-demand action initiated by the subscriber during the alerting phase.

Call Deflection Sequence

Call Deflection Configuration

Parameter	Type	Default	Description
cd_max_redirections	integer	5	Maximum number of sequential call deflections to prevent loops.

---

Multicall Configuration

Multicall allows a subscriber to maintain multiple simultaneous CS calls per 3GPP TS 23.135. This is distinct from MPTY (conference): in multicall, each call has independent audio. The subscriber switches between calls using the HOLD/RETRIEVE mechanism.

Parameter	Type	Default	Description
max_calls_per_subscriber	integer	2	Maximum number of simultaneous CS calls per subscriber. One call is active, remaining are held.
max_bearers_per_subscriber	integer	2	Maximum number of simultaneous radio bearers. Typically matches max_calls_per_subscriber.

---

eMLPP (Enhanced Multi-Level Precedence and Pre-emption)

eMLPP provides priority call handling per 3GPP TS 24.067. Calls are assigned a priority level, and higher-priority calls may pre-empt lower-priority calls when resources are scarce.

Priority Levels

Priority Level	Name	Description
0	A	Highest priority (Flash Override). Reserved for national emergency authorities.
1	B	Flash. Reserved for senior government/military officials.
2	0	Immediate. High-priority government/military.
3	1	Priority. General priority traffic.
4	2	Routine. Standard subscriber calls (default).

eMLPP Configuration

Parameter	Type	Default	Description
emlpp_enabled	boolean	false	Enable eMLPP priority call handling.
emlpp_default_priority	integer	4	Default priority level for calls without explicit priority (Routine).
emlpp_preemption_enabled	boolean	true	Whether higher-priority calls may pre-empt lower-priority active calls when resources are exhausted.
emlpp_preemption_tone	boolean	true	Play a preemption warning tone before disconnecting the pre-empted call.

---

AoCC (Advice of Charge - Charging)

AoCC provides real-time charge information to the subscriber during a call per 3GPP TS 24.086. The MSC sends charge rate information and accumulated charge indications to the mobile station.

AoCC Configuration

Parameter	Type	Default	Description
aocc_enabled	boolean	false	Enable Advice of Charge - Charging.
aocc_currency	string	"EUR"	Currency code (ISO 4217) for charge display.
aocc_rate_source	atom	:camel	Source of charging rates: :camel (from SCP via CAP), :local (from local rate table), :cdr (from CDR parameters).
aocc_update_interval	integer	10	Interval in seconds between charge update notifications to the subscriber.

---

3GPP Specification References

Specification	Title	Feature
TS 24.084	Multi-Party (MPTY) Supplementary Service	MPTY / Conference calling
TS 24.091	Explicit Call Transfer (ECT) Supplementary Service	ECT
TS 24.093	Call Completion to Busy Subscriber (CCBS)	CCBS
TS 23.135	Multicall	Multicall, CCBS network procedures
TS 24.072	Call Deflection Supplementary Service	Call Deflection
TS 24.067	enhanced Multi-Level Precedence and Pre-emption (eMLPP)	eMLPP priority levels
TS 24.086	Advice of Charge (AoC) Supplementary Services	AoCC charging
TS 24.083	Call Waiting and Call Hold	Hold/Retrieve for multicall