Inbound and outbound rules in 3CX
Set up inbound rules (send each DID to the right place) and outbound rules (which trunk and caller ID) in 3CX.
Once your SIP trunk is connected, 3CX needs to know two things: where to send each incoming call, and which trunk and caller ID to use when staff dial out. That is the job of inbound and outbound rules. Get them right and a customer dialling your sales number lands on the sales queue, your accounts team shows the accounts number on outbound calls, and nobody picks up the wrong line.
This guide explains what inbound rules (DID routing) and outbound rules do and the traps that catch most businesses. We will keep it conceptual rather than version-specific, because 3CX moves menus around between builds — but the logic below holds across versions.
Inbound rules: send each DID to the right place
A DID (Direct Inward Dialling number) is a phone number that points at your system — you might have one main number, a block of numbers, or a mix of geographic and 1300/1800 numbers. An inbound rule tells 3CX: when a call arrives on this number, send it here. The destination can be almost anything 3CX can ring:
- A single extension, or a ring group so several phones ring together
- A call queue with a hold position and a polling strategy
- A digital receptionist (IVR) that offers menu options
- A voicemail box, an external number, or a Call Flow Designer app
You create one inbound rule per DID. The cleanest setup matches each number to a clear purpose: main number to the receptionist or IVR, sales number to the sales queue, support number to support. The number a customer dials then tells your system how to treat the call.
Most inbound rules also need two destinations: where calls go during office hours, and where they go outside office hours. By day a call might hit a queue; after 5pm and on weekends it should go to voicemail or a mobile. 3CX lets each rule follow your business hours and public holidays and switch automatically — we cover that in business hours, holidays and after-hours routing.
Outbound rules: which trunk, and what number to show
When a staff member dials out, an outbound rule decides how that call leaves the building. It answers three questions:
Which calls does this rule apply to?
Match by the digits dialled (anything starting with 0 for an outside line, say), by number length, or by which extension or group is calling.
Which trunk carries the call?
Usually your SIP trunk, with an optional backup trunk so calls still get out if the first is unavailable.
What caller ID do we present?
The number the person you call sees — one main number for the whole company, or different caller IDs per department or extension.
3CX reads outbound rules top down and uses the first that matches, so order matters: put your most specific rules above the general catch-all. A common pattern is one rule for emergency and special numbers, one for mobiles and local calls, and a final rule for everything else.
Rules can also reshape the number before it reaches the trunk: strip digits removes leading digits (handy if staff dial a 0 for an outside line your trunk does not want), and prepend adds an area or country code so a short-dialled number goes out in full. If outbound calls get rejected, digit formatting is the usual culprit, and it is closely tied to how the trunk is configured — see connecting a SIP trunk to 3CX.
Caller ID, and a word on security
Customers are far more likely to answer a recognisable business number, and the correct department number means return calls land in the right place. One honest caution: you can generally only present numbers you own or are authorised to use, so if you want to display a 1300 or 1800 number, make sure it is hosted or ported first.
Outbound rules are also a front line against toll fraud. Rules that let any extension dial any international destination are exactly what attackers exploit, so restrict international and premium dialling to the extensions that need it and lean on 3CX's anti-fraud limits. If this part makes you uneasy, that is a reasonable instinct — it is where a small mistake gets expensive fastest.
Getting the setup right
Rules are not hard once you see the logic, but a real business has more moving parts than a guide can cover — multiple numbers, departments, after-hours flows and security limits all interacting. Setting them up correctly is part of every 3CX system we install. As a local Melbourne telco we are happy to map your numbers and routing with you — call 1300 747 266 or get in touch and we will sort it.
What we see installing this in Melbourne
On almost every Melbourne cutover, the bit that bites is digit formatting on outbound rules. Australian SIP carriers vary on whether they want the leading 0, full E.164 (+61), or a stripped national format, and a rule that worked on the old PBX often needs a strip-or-prepend tweak before mobile and 13/1300 calls behave. We test every category from a real extension before we leave — local, mobile, 1300, international, 000.
Caller ID is the other one. SMEs often want the main number on every outbound call, then discover sales callbacks land at reception. We map caller IDs per department on the day, and check porting status on any 1300/1800 number first — if it has not landed on the new trunk yet, presenting it just fails silently.
After-hours destinations are where we add the most value post-cutover: walking the office manager through editing the holiday list and after-hours mobile themselves, so they are not ringing us every long weekend.
Frequently asked questions
What is the difference between a DID and an inbound rule?
A DID is the phone number that points at your system. An inbound rule is the instruction that says where calls on that number go — to an extension, a queue, an IVR and so on. You normally have one inbound rule per DID.
Can different staff or departments show different caller IDs?
Yes. Outbound rules and extension settings let you present a single company number for everyone, or different numbers per department or person, as long as you are authorised to use those numbers. This is common when sales and accounts want callbacks to reach the right team.
Why do my outbound calls fail or get rejected?
The most frequent cause is number formatting in the outbound rule — too many or too few digits for what your carrier expects. Stripping a leading 0 or prepending an area or country code usually fixes it. The trunk's own configuration matters too, so check the two together.
How do calls route after hours?
Each inbound rule can have separate destinations for office hours and outside office hours, and 3CX switches between them automatically based on your business hours and public holidays. After-hours calls typically go to voicemail or a mobile.
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