Telecoms glossary • P
PSTN (Public Switched Telephone Network)
The copper and ISDN network business telephony grew up on. VoIP now accounts for more than half of South Africa’s remaining fixed-line subscriptions.
What is the PSTN?in VoIP and business telephony
1. The PSTN is the circuit-switched telephone network that carried voice before IP: copper pairs, exchanges and a dedicated channel held open for the whole duration of a call. In South Africa it means Telkom’s legacy copper and ISDN estate, and it is shrinking faster each year.
2. Circuit switching is the defining difference. A PSTN call reserves an end-to-end path whether anyone is speaking or not. A VoIP call sends packets only when there is something to send, and shares the line with everything else the business does.
How circuit switching differs from packets
A PSTN call is a reservation. When the call is set up, a 64 kbit/s channel is allocated end to end and held until someone hangs up, whether the parties are talking, listening or silent. The quality is consistent because nothing else is allowed to use that channel.
The 64 kbit/s figure is not arbitrary. It is G.711 pulse code modulation sampling the voice band 8,000 times a second at eight bits a sample, which is also why traditional calls sound narrower than HD voice: G.722 carries seven kilohertz of audio in the same 64 kbit/s, and the PSTN never did.
Those channels are bundled for business use. An E1 circuit carries 32 timeslots of which 30 are usable voice channels, with one timeslot for framing and one for signalling. That is where the familiar thirty-channel ISDN primary rate line comes from, and why capacity on the old network was bought in fixed blocks rather than the flexible channel counts a SIP trunk offers.
The trade-off is the whole argument. Reserved capacity means predictable quality and guaranteed limits. Packet switching means capacity is shared, which is cheaper and more flexible but makes QoS your responsibility rather than the network’s.
What the numbers say about the shift
ICASA publishes the counts annually, and they are unambiguous. In the year to 30 September 2025, total fixed-line subscriptions in South Africa fell 12.9%, from 954,952 to 832,073.
| Fixed-line subscriptions | 30 Sept 2024 | 30 Sept 2025 |
|---|---|---|
| Analogue telephone lines | 282,505 | 115,689 |
| ISDN voice-channel equivalents | 246,010 | 214,470 |
| VoIP subscriptions | 389,041 | 469,130 |
| Total | 954,952 | 832,073 |
Two lines in that table do the work. Analogue lines fell by roughly 59% in a single year. VoIP subscriptions grew 20.6% and now make up more than half of every fixed-line subscription in the country. The PSTN is not being argued about any more; it is being counted down.
If your business is still on copper or ISDN
The migration question is not whether, it is what to replace it with, and the honest answer depends on what you have already paid for. Our guide to moving away from Telkom covers the sequencing in detail.
- Replace the system entirely
- Move to a Cloud PBX, retire the on-site hardware and pay per user. The right answer for most businesses, and the only one that also solves the maintenance contract on a PBX nobody can get parts for.
- Keep the PBX, replace the lines
- SIP trunking connects an existing on-premise PBX to the outside world over your internet connection. Sensible where the PBX is recent, capable and paid for. Channels are bought per concurrent call, not per user, which is usually the saving.
Whichever route you take, deal with the numbers first. Number portability lets you carry your existing numbers across, and an account in arrears is a valid ground for refusing the port, so settle before you lodge and never cancel the old service until the port is confirmed.
Audit the sockets before you cancel anything
Walk the building and find every analogue termination: the lift phone, the alarm panel, the gate intercom, the fax machine in accounts, the card terminal at the counter. Each one either needs an analogue port on the new system or a replacement, and each one is a bad thing to discover on cut-over day.
Then check what the line is actually costing against what the alternative costs, using concurrent calls rather than line count as the measure. A thirty-channel ISDN primary rate that peaks at eight simultaneous calls is a straightforward saving. Compare VoIP providers on channel pricing rather than headline per-user rates when that is the shape of your usage.
About this entry
Definitions are written for South African business buyers and checked against primary sources – provider documentation, standards bodies and ICASA – not vendor marketing. Reviews are independent and sponsors are always disclosed. Read our editorial policy and scoring methodology.
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