Numbers & trunks
PBXAlso known as Private Branch Exchange, IP PBX, Cloud PBX
Definition
A PBX (private branch exchange) is a private telephone system that connects an organisation’s internal extensions to each other and to the outside world through trunks. Modern IP PBXs, such as Asterisk, FreePBX and 3CX, use SIP for phones and trunks and add features like IVR, voicemail, queues and call recording.
What a PBX does
- Internal calling between extensions at no cost.
- Inbound routing of DIDs to users, ring groups, queues or an IVR.
- Outbound routing to the best SIP trunk or gateway by number, time or cost, with failover.
- Features: voicemail, transfer, hold music, conferencing, call recording and CDRs.
Types of PBX
| Type | Where it runs | Notes |
|---|---|---|
| Legacy / TDM PBX | On-site hardware | Analogue or digital handsets, ISDN trunks |
| IP PBX | On-site server or VM | SIP phones and trunks; Asterisk, FreePBX, 3CX |
| Hosted / cloud PBX | Provider’s data centre | Subscription per user; no local server |
A PBX is a Class 5-style system on a small scale: it serves end users. Carriers’ softswitches do the same at much larger scale.
Connecting a PBX to mobile lines
Choosing a PBX usually comes down to scale, features and who will maintain it. Open-source systems such as Asterisk and FreePBX are flexible and free to license but need in-house skills; commercial IP PBXs and cloud services trade some flexibility for vendor support and simpler administration.
Adding a GSM gateway as a trunk lets a PBX call mobile numbers over mobile tariffs, receive calls to mobile numbers and fall back to mobile lines if fixed trunks fail. GSMCalls works with standard SIP PBXs; see the guides for FreePBX, Asterisk and 3CX.
Frequently asked questions
What is the difference between a PBX and a SIP trunk?
The PBX is the phone system that manages extensions and routing. The SIP trunk is the connection that carries its external calls.
Is Asterisk a PBX?
Asterisk is an open-source communications engine commonly used as an IP PBX; FreePBX is a web interface and distribution built on it.
Do I need an on-site PBX?
Not necessarily. Cloud PBXs provide the same features as a service, while on-site IP PBXs give more control.