SIP for BYO PABX: What You Need to Know

If your business has a phone system that still works well, replacing it just because the old lines are gone can feel like a waste. Connecting a bring your own (BYO) PABX to SIP lets you keep the system, the handsets and the way your team already works, while your calls move onto the internet. It does involve a few checks first, and knowing them upfront saves a lot of troubleshooting later.

What is SIP for BYO PABX?

SIP for BYO PABX means connecting your existing on-premises phone system to SIP trunks, so calls travel over your internet connection instead of old ISDN or copper lines. You keep your PABX, handsets, extensions and call setup — only the lines connecting your system to the outside world change.

Think of your PABX as the switchboard in the comms cupboard, handling extensions, hunt groups, voicemail and transfers. The SIP trunk connects that switchboard to the public phone network. In the past, that connection was ISDN or a bundle of analogue lines. ISDN has been phased out in Australia, and copper phone lines in NBN areas have been retired, so SIP is how most on-premises systems now reach the outside world.

Each trunk is split into channels, and each channel carries one call at a time. Five channels means five calls at once, whether incoming or outgoing.

Will my PABX work with SIP?

Many PABX systems sold in the last 10 to 15 years can work with SIP, often after adding a SIP licence or upgrading the firmware. Older systems built only for ISDN or analogue lines can usually still connect through a VoIP gateway, a small device that converts SIP into the line type the PABX expects.

Here is how that tends to play out:

• IP-capable systems (such as many NEC SV and SL series, Panasonic KX-NS and NSX, Avaya IP Office and Mitel models) usually support SIP natively. You may need to buy SIP trunk licences from the manufacturer, and the system may need a firmware update first.
• Older digital systems that only have ISDN or analogue ports can connect through a gateway. The PABX thinks it is still talking to ISDN lines, while the gateway handles SIP on the other side.
• Very old or unsupported systems can sometimes be connected, but parts, firmware and technical support may be hard to find. This is where replacing the system starts to make more sense.

Tip: Before you call anyone, find the make and model of your PABX (it is usually printed on the main unit) and check whether you still have the admin login. Both come up straight away in any SIP conversation, and a missing password can slow things down.

What changes when you move your PABX to SIP?

Your staff should notice very little. Extensions, handsets, voicemail and transfers stay the same, and the changes mostly happen behind the scenes.

• Your numbers move across. Your existing phone numbers, including any direct-dial number ranges from an old ISDN service, can generally be ported to the SIP provider. Porting usually takes a few days to a few weeks, depending on the number type and the losing carrier, so it is worth planning the cut-over date around it.
• Your internet connection carries the calls. That means it needs enough upload speed and a little care in how it is set up (more on this below).
• Your call costs change. Rather than paying line rental per ISDN service, you pay per SIP channel plus your call charges, or a fixed amount on an unlimited plan.

The technical details worth getting right

Most SIP problems on a BYO PABX come down to a handful of settings. Your installer should handle these, but it helps to know what they are.

Codecs

The codec is the format used to carry voice. Australia uses G.711 A-law, while North America uses G.711 μ-law. A PABX with factory settings from overseas, or a mismatch between the PABX and the provider, can cause distorted audio or failed calls. Matching the codec on both sides is one of the first things to check.

Router settings and SIP ALG

Many business routers have a feature called SIP ALG that tries to “help” SIP traffic through the firewall. In practice, it often rewrites call information incorrectly, causing one-way audio, dropped calls or phones that stop receiving calls. Most SIP providers recommend turning it off. Your router also needs the right firewall rules so the provider’s servers can reach your PABX, and so nobody else can.

Bandwidth and Quality of Service

Each call on G.711 uses roughly 100 kbps in each direction. Ten calls at once need about 1 Mbps of upload and download set aside for voice. That is modest, but upload speed is often the weak spot on standard NBN plans. Quality of Service (QoS) on your router keeps voice ahead of cloud backups and video meetings.

Registration or IP authentication

Your PABX connects to the provider either with a username and password (registration) or by its fixed IP address (IP authentication). Some models only support one method, so confirm which yours uses before choosing a plan.

Don’t forget the devices plugged into your PABX

This is the step that catches businesses out. Over the years, plenty of things end up connected to the phone system, and not all of them behave well over SIP.

• Fax machines can be unreliable over SIP. Fax to email is often the simpler option.
• EFTPOS terminals that dial out over a phone line may need to move to their internet or mobile connection.
• Security alarms and lift emergency phones often rely on a phone line to call a monitoring centre. Check with the alarm or lift provider before cut-over, as these devices can have specific requirements.
• Door phones and paging systems on analogue ports usually keep working, but test them.

Walk through the comms cupboard and list everything with a phone cable in it. It takes 15 minutes and avoids an alarm that quietly stops reporting.

How many SIP channels does a PABX need?

A PABX needs enough SIP channels to cover the most calls happening at once during your busiest period, not one per staff member. If you are moving from ISDN, your old service is a useful starting point, though many businesses find they were paying for more capacity than they used.

Your PABX call logs are the best guide. Look at peak concurrent calls during your busiest hour, then add one or two channels as a buffer. Remember that a call transferred to an outside number, like a staff mobile, usually uses two channels: one for the incoming call and one for the outgoing leg.

For businesses still working out their call volumes, a PAYG SIP trunk keeps things flexible. You pay a small monthly fee per channel plus your calls, and can add or remove channels as you learn what you actually need.

Should you keep your PABX or replace it?

The table is a starting point, not a verdict. A well-maintained SIP PABX can keep running reliably for years. But if your system is no longer supported by the manufacturer, or parts are getting hard to source, spending on licences and gateways may only delay a replacement you will need anyway, whether that is a new on-site system or a cloud phone system.

Security and emergency calls

Once your PABX is reachable over the internet, it needs the same care as any other business system. Toll fraud, where someone gains access to a phone system and uses it to make expensive international calls, is a real risk for poorly secured systems. Change default passwords, restrict access to your provider’s servers, and ask about international call barring and spend alerts.

For emergency calls, a SIP PABX needs power and internet to dial 000. A UPS for the PABX and the NBN connection box help during short outages, and a charged mobile on site covers longer ones. Make sure each number has the correct street address registered, so emergency services are sent to the right place.

Getting started

If you are weighing up SIP for your current system, start with three things: the make and model of your PABX, a list of everything plugged into it, and your peak call volume. With those in hand, a provider can tell you fairly quickly whether you need licences, a gateway or a rethink.

Com2’s SIP trunks are set up to work with a wide range of phone systems, and the team can look after the licensing, porting and programming needed to move your existing system across.