SIP Trunking vs ISDN: Which Should Your SA Business Use in 2026?
If your business still runs on ISDN lines, you’re using technology that’s actively being switched off — and the replacement, SIP trunking, is cheaper and more flexible anyway. This guide compares the two head to head, explains why the change is happening now, and helps you plan the move.
The quick definitions
ISDN (Integrated Services Digital Network) is a decades-old standard that delivers voice (and some data) over digital phone lines. For years it was the backbone of business telephony in South Africa, sold in fixed bundles of channels.
SIP trunking delivers the same voice service — making and receiving calls to the public phone network — but over your internet connection instead of dedicated ISDN lines. (See SIP Trunking Explained for the full picture.)
Why this comparison matters now
This isn’t a theoretical debate. Telkom is decommissioning its copper network, and ISDN is being phased out as part of that retirement. Businesses on ISDN face rising costs, harder-to-source support, and an eventual hard stop. Planning the move to SIP now — on your terms — is far better than being forced off ISDN at short notice.
Head-to-head comparison
| Factor | ISDN | SIP Trunking |
|---|---|---|
| Underlying technology | Legacy digital phone lines | Voice over your internet connection |
| Future | Being retired | The modern standard |
| Cost | Higher line rental + call rates | Lower per-channel + cheaper calls |
| Scalability | Fixed bundles; slow to change | Add/remove channels easily |
| Setup | Physical lines, lead times | Software-based, fast |
| Resilience | Tied to the physical line | Reroute to mobile/another site |
| Number flexibility | Limited | Keep and move numbers freely |
Cost
ISDN carries ongoing line rental plus call charges at legacy rates. SIP trunking typically combines a small monthly per-channel fee with much lower call rates — in the SA market roughly 24–30c/min to landlines and around 79c/min to mobiles, often bundled. For most businesses the switch reduces the monthly phone bill noticeably while adding capability. (See Cloud PBX Pricing in South Africa 2026.)
Quality and reliability
ISDN was prized for consistent voice quality, and that reputation made some businesses wary of “internet calls.” But on a decent modern connection with a quality SIP provider — and voice traffic properly prioritised — SIP quality matches or beats ISDN. Crucially, SIP adds resilience ISDN never had: if your site or line fails, calls reroute automatically to mobiles or another location, so you stay reachable.
Scalability
This is where SIP wins decisively. ISDN comes in fixed bundles (e.g., 2, 10, 30 channels), and changing capacity means provisioning physical lines with lead times. SIP channels are software — scale up for a busy season and back down afterwards, in days not weeks, with no rewiring.
What about call quality concerns?
The old objection to SIP was unreliable internet. In 2026, with widespread fibre and fixed-LTE/5G, that concern has largely evaporated for most businesses. The keys are adequate bandwidth, a quality provider, and a sensible backup connection — all easily arranged. Meanwhile, staying on ISDN is the genuinely risky choice, given it’s being switched off.
Planning your move from ISDN to SIP
- Audit your current lines, numbers and call volumes.
- Check your internet — confirm bandwidth and consider a fixed-LTE backup.
- Choose an ICASA-licensed SIP provider with local support.
- Port your numbers so customers notice no change (typically 5–10 business days).
- Right-size your channels to your concurrent-call needs, with room to scale.
- Test and cut over, keeping a fallback until you’re confident.
A capable provider runs this for you with minimal disruption — often you keep working throughout.
Frequently asked questions
Do I have to leave ISDN?
Eventually, yes — it’s being retired. Moving on your own schedule is far better than being forced off at short notice.
Will I keep my business numbers?
Yes. Your existing numbers port to SIP, so nothing changes for your customers.
Is SIP call quality really as good as ISDN?
On a decent connection with a quality provider and voice prioritisation, yes — typically as good or better, with added resilience.
Do I need a new phone system to use SIP?
Not necessarily. A SIP-compatible on-site PBX can use a SIP trunk, or you can move to a cloud PBX that includes it.
Plan your move off ISDN
NexysCloud handles ISDN-to-SIP migrations for South African businesses — porting, sizing and cutover managed for you.
Book a consultation or call 010 110 7100.
