What does ACD stand for?

Automatic call distributor. It is the system that holds incoming calls in queues and distributes them to agents according to configured rules, as distinct from an IVR, which collects input from the caller to decide where they should go in the first place.

Can you combine routing strategies?

Commonly, yes. Skill-based routing and sticky routing act as filters that narrow the eligible pool, and an underlying strategy such as round robin or longest idle then chooses within it.

What happens if no agent is available?

The call waits, subject to the queue's maximum wait time. When that is reached it should overflow to a backup queue, go to voicemail, or be offered a callback — the failure mode to avoid is holding people with no exit path.

How is ACD routing different from an IVR?

An IVR interacts with the caller — a menu, a prompt, sometimes an AI receptionist — to determine intent and pick a destination. The ACD takes it from there, holding the call and selecting an agent. Most inbound flows use both in sequence.

Voice & ACD

ACD call routing strategies explained

An ACD (automatic call distributor) routing strategy is the rule a call queue uses to choose which available agent receives the next waiting call. The common strategies are round robin (even distribution), least occupied (fewest calls handled), longest idle (waiting longest), skill-based (only agents meeting a required proficiency), and sticky (back to the agent who last handled that caller).

7 min readUpdated 5 August 2026

The five strategies, and what each optimises

Round robin: Cycles through agents in order. Optimises for even distribution of volume. Simple and predictable, and fair in the narrow sense that everyone gets the same number of calls — regardless of how demanding those calls were.
Least occupied: Goes to whoever has handled the fewest calls. Optimises for balanced workload rather than balanced count, and self-corrects when agents join or leave a shift mid-day.
Longest idle: Goes to whoever has been waiting the longest since their last call. Optimises for responsiveness and tends to feel fairest to agents, since sitting idle is what gets rewarded rather than finishing calls quickly.
Skill-based: Only offers the call to agents who meet the queue's required skill and proficiency level. Optimises for first-contact resolution at the cost of a smaller eligible pool — and therefore longer waits.
Sticky: Routes a returning caller back to the agent who handled them last, within a time window. Optimises for continuity: the customer does not re-explain their problem.

Which routing strategy should you use?

Situation	Strategy	Why
Uniform work, interchangeable agents	Round robin	Nothing to optimise beyond fairness of volume
Calls vary a lot in length	Least occupied	Count-based fairness misallocates when handle times differ
Service level is the priority	Longest idle	Minimises the time a ready agent sits unused
Tiered support or specialisms	Skill-based	Getting the right person first time beats answering faster
Ongoing cases, collections, account management	Sticky	Continuity is worth more to the customer than speed
Mixed floor	Skill-based over another strategy	Filter to the eligible pool, then cycle within it

How do agent skills and proficiency levels work?

Skill-based routing needs two pieces of data: what each agent is capable of, and what each queue demands. Skills are typically rated on a scale — for example 1 to 5 — per agent per skill, and a queue specifies a minimum level for one or more skills.

The value of levels rather than a yes/no flag is that a tier-two billing question stops landing on someone who was marked 'billing' on their first day. It also gives you an escalation path that does not require separate queues for every gradation of competence.

The queue settings that matter as much as the strategy

Maximum wait time: How long a caller may wait before something else happens. Without a ceiling, a queue with no available agents simply holds people indefinitely.
Overflow queue: Where calls go when the ceiling is reached — usually a broader team with looser skill requirements. This is what turns a wait limit into a plan rather than a disconnect.
Business hours: Per-queue operating schedules, so out-of-hours calls are handled deliberately instead of ringing an empty floor.
Hold music: Per-queue audio. Minor, until you hear the default tone your customers have been listening to for months.
Wrap-up time: A protected period after each call for notes and disposition. Without it, agents get a new call mid-sentence and quality drops in ways routing changes cannot fix.
Agent states: Ready, on break, logged out. Routing can only be as good as the availability signal it is working from.

Frequently asked questions

What does ACD stand for?: Automatic call distributor. It is the system that holds incoming calls in queues and distributes them to agents according to configured rules, as distinct from an IVR, which collects input from the caller to decide where they should go in the first place.
Can you combine routing strategies?: Commonly, yes. Skill-based routing and sticky routing act as filters that narrow the eligible pool, and an underlying strategy such as round robin or longest idle then chooses within it.
What happens if no agent is available?: The call waits, subject to the queue's maximum wait time. When that is reached it should overflow to a backup queue, go to voicemail, or be offered a callback — the failure mode to avoid is holding people with no exit path.
How is ACD routing different from an IVR?: An IVR interacts with the caller — a menu, a prompt, sometimes an AI receptionist — to determine intent and pick a destination. The ACD takes it from there, holding the call and selecting an agent. Most inbound flows use both in sequence.

How Anzzai does this

Voice & ACDAll five routing strategies, skills with proficiency levels, overflow, business hours and hold music in Anzzai.