IP PBX: definition and a practical example
Short answer: An IP PBX is a company's private telephone exchange running over an IP network, connecting extensions, trunks and IVR and routing internal and external calls with SIP and RTP.
In Portuguese: PABX IP
What is an IP PBX?
An IP PBX is a company’s private telephone exchange that uses the data network instead of telephone wiring. It registers extensions (IP phones, softphones and adapters for analog handsets), connects the company to the carrier through SIP trunks or, via a gateway, E1 trunks, and decides where each call goes.
Signaling between extensions, the PBX and the carrier uses SIP, and audio travels in RTP packets. On top of that, the PBX offers the usual features: IVR, call queues, transfer, call forwarding, voicemail, recording and the mapping of DID numbers to extensions.
An IP PBX can be a server or appliance on the company’s premises, a virtual machine in a data center or a hosted service. In every case, it is only one part of the path. A call coming from outside goes through the carrier, the trunk, the firewall or SBC, the PBX, and only then reaches the extension or IVR. Monitoring the PBX’s CPU, memory and processes shows that the server is on, but not that a customer’s call completes with audio in both directions.
In practice
An office with 40 extensions, a SIP trunk with 10 channels and a DID range from 11 3456-7000 to 7039. An outside call to 11 3456-7012 goes through:
| Segment | What can fail |
|---|---|
| Carrier | Number routing, number portability, channels |
| SIP trunk | Registration, authentication, channel limit |
| Firewall or SBC | NAT, SIP ALG, blocked RTP ports |
| IP PBX | Dial plan, DID mapping, codecs |
| Extension 7012 | Extension unregistered, phone switched off |
With the PBX up, any one of these segments can drop the call.
How Monitor uses this
Monitor works with any PBX because it calls numbers, just as a customer would. There is no integration, agent or configuration on the PBX: the test dials the public number, and the result shows whether the call completed across the whole path. See SIP trunk monitoring.
Frequently asked questions
Do I need to integrate call monitoring with my IP PBX?
Not when the test is done with real calls. A test call dials the public number, just as a customer would, and travels through the carrier, the trunk, the PBX and the extension or IVR. So it does not depend on the PBX's brand, version or API, and it also catches failures outside the PBX, such as routing at the carrier or problems on the internet link.
What is the difference between an IP PBX and a cloud PBX?
The function is the same: extensions, queues, IVR and routing. With an on-premises IP PBX, the server sits at the company and the team takes care of hardware, updates and the link. With a cloud PBX, the provider hosts the system and extensions register over the internet. Either way, the call depends on the trunk, the network and the configuration, and it can fail even while the PBX is up.