RustPBX Basics: Queues & ACD — Fairness, Priority, and the Overflow Ladder

Queues are where callers decide whether your company has its act together. The ACD (Automatic Call Distribution) engine has to balance three promises: fairness (first come, first served), priority (VIPs really do go first), and escape (nobody waits forever). Here’s how RustPBX delivers each.

Strategy: Picking the Agent

Queues console with hold treatment, targets and fallback

Five assignment algorithms cover the standard rotations:

Algorithm	Behavior
`longest_idle`	Longest-idle agent first (default)
`least_answered`	Protects new agents
`round_robin`	Even distribution
`skill_based`	Weighted scoring: `skill_match × 0.6 + idle_time × 0.3 + priority × 0.1`
`random`	Special cases

Skill weights are configurable per policy, and require_exact_skill toggles between “at least one skill” and “all skills” matching.

ACD queue configuration with strategies and overflow

Priority Without Starvation

Every waiting call has a score: base_priority + vip_bonus + wait_time × weight. VIP bonuses are mapped by tier, and the wait-time weight means a caller’s score climbs every second — so a silver customer waiting 5 minutes outranks a gold customer who just called. Within the same priority, dispatch is FIFO, and in clusters the FIFO gate is strict across nodes.

Overflow: The Ladder

Overflow triggers (timeout / queue length / no agents) push calls down a chain: another skill group, voicemail, callback, back-to-IVR, or hangup. Three modes decide how the chain is walked:

replace — hand off to the next group
cumulative — ring original + overflow groups together
sequential — dwell in the current queue, then re-queue into the next group in order

sequential is the polite one: agents downstream only get interrupted for calls that genuinely ran out of time upstream. Every hand-off emits an overflow_joined event.

The Comfort Layer

Queue hold music as a PBX-wide default (queue_hold_music), overridable per queue.
Screen-pop headers (Call-Info / User-to-User) re-applied on every re-ring — see the screen-pop post.
Queue-entry traces with the resolved agent, so each dispatch decision is auditable.

Watching It Live

Realtime monitor with waiting calls, skill groups and live call stages

GET /cc/realtime and the monitor page show per-queue waiting depth, longest wait, and agent states — reconciled against the shared queue tracker, not a single in-memory counter. Full configuration: Queues & ACD.

A Real-World Recipe: The VIP Line

Putting it together — a VIP number that must never wait long:

A route rule matches the VIP DID and sets base_priority via queue metadata
The queue uses skill_based with require_exact_skill = true (only fully-trained agents)
Overflow after 30s → senior group, sequential mode, then voicemail
Screen-pop headers carry the customer id; CSAT survey with after-transfer opt-in

Four config blocks, no code — and every step above links to a full guide in the docs.