A phone system should not hold your business back when you open a second site, add remote staff or need to respond quickly to customers. The SIP vs PSTN decision is now especially relevant for Australian businesses as traditional copper phone services are progressively retired and more organisations move their calling to internet-based platforms.
PSTN has served Australian homes and businesses for decades. It is familiar, straightforward and was built around dedicated physical phone lines. SIP takes a different approach: calls travel over an internet connection, making it possible to run business numbers, handsets and softphones through a cloud phone system.
Neither option is automatically right for every situation. The practical choice depends on your existing infrastructure, call volumes, internet connection, continuity requirements and plans for the next few years.
SIP vs PSTN: the practical difference
PSTN stands for Public Switched Telephone Network. It is the traditional telephone network that carried voice calls over copper lines and exchanges. A PSTN service typically provides a fixed number connected to a physical location, with each line supporting one call at a time.
SIP stands for Session Initiation Protocol. In business telephony, SIP is the technology used to establish and manage voice calls over an IP network. A SIP trunk can connect your business phone system to the internet, while cloud PBX platforms use SIP to provide calling features without requiring a traditional on-site phone exchange.
The distinction matters because PSTN is tied closely to physical infrastructure, whereas SIP is designed for connected workplaces. A team member can use a desk handset in the office, an app on their mobile or a softphone on their laptop while keeping the same business number and extension.
For a small local business with one premises and simple calling needs, that flexibility may not be the deciding factor. For a growing business with multiple locations, hybrid staff or customer-facing teams, it can change how easily calls are handled each day.
Why Australian businesses are moving away from PSTN
The transition is not only about adding new features. Much of Australia’s older copper-based telephone infrastructure is being decommissioned as services shift to the NBN and IP-based alternatives. Businesses still relying on older analogue lines should assess their replacement options before a service change becomes urgent.
A PSTN line was traditionally valued for its independence from the office internet service. However, it also has limitations: capacity is fixed, moves and changes can take longer, and expanding often means ordering and installing additional lines. Older services may also support important equipment such as alarms, EFTPOS terminals, lift phones, fax machines and monitored devices. These need to be checked individually during any migration.
SIP removes many of the scaling constraints. Instead of installing another physical line for every additional call path, businesses can generally add capacity through their provider and phone system configuration. That makes it more suitable for seasonal peaks, expanding sales teams and multi-site operations.
Call quality and reliability: internet quality matters
The main concern many businesses raise about SIP is call quality. This is a fair question. SIP calls rely on your internet connection, so poor bandwidth, network congestion, unstable Wi-Fi or an incorrectly configured router can result in delay, jitter or dropped calls.
With the right connection and setup, SIP voice quality can be clear and consistent. The priority is not simply choosing the fastest plan available. It is ensuring there is enough reliable upload and download capacity for concurrent calls, alongside cloud applications, video meetings, backups and everyday staff traffic.
For example, a five-person office making occasional calls has very different requirements from a 30-seat customer service team handling calls throughout the day. Businesses with high call volumes may benefit from dedicated connectivity, traffic prioritisation through quality of service settings, managed networking or a separate connection for critical voice traffic.
Power continuity also needs consideration. Traditional phones could often continue working during a local power outage because power was supplied over the copper network. Most SIP handsets, routers and cloud phone equipment require local power. A UPS can keep essential equipment operating through short outages, while businesses with stricter continuity requirements may need backup internet, 4G or 5G failover, or a broader business continuity plan.
Reliability is not about choosing old technology over new technology. It is about designing the service around the risk your business can tolerate.
Cost, capacity and day-to-day management
PSTN costs can be simple to understand when only one or two lines are involved. But costs may become less attractive as a business adds lines, call capacity and separate services across multiple premises. Physical line limitations can also make a growing business pay for capacity that is not always used efficiently.
SIP usually provides greater flexibility. Businesses can choose the number of concurrent calls they need, retain existing local numbers in many cases, and scale up as staff or call volume grows. Calls between extensions, sites and remote workers can often be managed as part of the same phone environment.
The saving is not always just in call charges. It can come from reduced administration. Moves, new extensions, hunt groups, call forwarding, time-of-day rules and voicemail settings can be handled centrally rather than requiring physical changes to a phone line.
That said, SIP pricing should be assessed properly. Ask what is included in the monthly service, whether call packs suit your usage, how additional call paths are charged, and whether compatible handsets or a cloud PBX subscription are required. The lowest advertised price is not necessarily the best value if it lacks local support or the features your team relies on.
Features that change how your team answers calls
PSTN was built to make dependable voice calls. SIP can do that while adding tools that suit modern work practices. Depending on the phone system, useful functions can include auto attendants, call queues, ring groups, call recording, voicemail-to-email, shared reception consoles, reporting and mobile apps.
These features are valuable when they solve a real operational problem. A medical practice may need calls to queue rather than ring out. A trade business may want the office number to ring selected mobiles when staff are on the road. A multi-site retailer may need one main number with calls directed to the right location.
For businesses with straightforward requirements, it is still wise to keep the setup simple. More features do not automatically improve customer service. Clear call routing, reliable handsets and staff who know how to transfer a call properly will usually matter more than a long list of options.
Choosing between SIP and PSTN
For most Australian businesses planning ahead, SIP is the more practical long-term path. It supports flexible work, can grow with the business and aligns with the move away from legacy copper services.
PSTN may still be present in older sites or connected to specialised equipment, but it should not be assumed that every device can simply be plugged into a new internet service. Before changing anything, document every service using the existing lines. Include phones, security alarms, payment terminals, intercoms, fax services, medical devices, emergency phones and lift communications.
Then consider the connection underneath the phone system. NBN may be suitable for many offices, while private fibre, enterprise ethernet, fixed wireless or a backup mobile service may be appropriate where uptime and performance are more demanding. The right answer depends on location, available infrastructure and the cost of a missed call or extended outage.
Planning a smooth move to SIP
A successful migration starts with a short discovery process rather than a rushed port request. Confirm which numbers need to be retained, how many simultaneous calls the business needs, where staff work, and what should happen if the primary internet connection fails.
It is also worth testing the new service before switching completely where possible. Check call quality from every site, confirm inbound and outbound calling rules, and make sure staff can use core functions such as transfers, hold and voicemail. If your business takes a high volume of customer calls, schedule the final change outside peak operating hours.
Local support can make a meaningful difference here. A provider that understands both the internet connection and the phone system can help identify whether an issue sits with the network, handset, router configuration or call platform. InfiNET Broadband can bring connectivity and business voice services together, helping Australian businesses build a setup that is easier to manage as needs change.
The best time to review your phone system is before a legacy service becomes a problem. Start with how your team works and what your customers expect when they call, then choose a service designed to keep those conversations moving.