Quick summary: SIP trunking, ISDN, and PSTN are three different ways of connecting a business phone system to the public telephone network. PSTN and ISDN use physical copper lines — infrastructure that’s being phased out across Ireland and the UK. SIP trunking uses the internet instead, and is the technology underpinning modern cloud phone systems. Dromlik, a cloud phone system for Irish businesses, is built on SIP technology — giving your business Irish local numbers, intelligent call routing, mobile apps, and an AI receptionist, without any of the complexity of managing SIP infrastructure yourself. Plans start from €12/user/month (annual billing).
What Is the PSTN?
The Public Switched Telephone Network (PSTN) is the traditional analogue telephone network — the copper wire infrastructure that has connected Irish businesses to the phone network for over a century. When you plug a phone into a standard wall socket, that’s the PSTN.
For businesses, a basic PSTN connection means one phone line per physical wire. If you want two simultaneous calls, you need two lines. If you want ten, you need ten. Each line has a monthly rental cost, and the infrastructure is fixed to your premises — if you move, your number doesn’t automatically come with you.
What Is ISDN?
ISDN (Integrated Services Digital Network) was developed in the 1980s as a digital upgrade to the analogue PSTN. It carries voice and data over copper lines, but more efficiently — each ISDN channel carries one call, and channels are bundled:
- ISDN2 (BRI) — 2 channels, supporting 2 simultaneous calls
- ISDN30 (PRI) — up to 30 channels, supporting up to 30 simultaneous calls
ISDN became the standard for Irish businesses with on-site PBX systems — the PBX connects to an ISDN line, which connects to the telephone exchange. It’s more capable than basic PSTN but still tied to physical copper infrastructure.
The problem with ISDN in 2026: It’s being phased out. BT Openreach has switched off ISDN in Northern Ireland as part of the January 2027 PSTN shutdown. In the Republic, Eir and other providers are deprecating ISDN infrastructure across the country. Businesses still on ISDN need to migrate.
BT Openreach’s PSTN and ISDN switch-off deadline for Northern Ireland is 31 January 2027. In the Republic, ISDN is being phased out by providers including Eir. If your business is still on an ISDN line, the clock is running.
What Is SIP Trunking?
SIP (Session Initiation Protocol) is the technology standard that manages voice calls over IP networks — in other words, over the internet. A SIP trunk is a virtual phone line that connects a phone system to the public telephone network via the internet, rather than via a physical copper line.
Where ISDN uses copper wires and physical channels, SIP uses internet data packets. The result:
- No physical line required — calls travel over your existing broadband connection
- Scalable on demand — need more concurrent call capacity? Add it instantly, no engineer visit
- Not tied to a location — a SIP number follows the internet connection, not a physical address
- Cheaper than ISDN — no expensive line rental per channel
SIP is the technology that makes cloud phone systems possible. When you make a call on Dromlik, WhatsApp, or any modern VoIP app, the underlying technology is SIP.
PSTN vs ISDN vs SIP: What’s the Difference?
| PSTN | ISDN | SIP / VoIP | |
|---|---|---|---|
| Technology | Analogue copper | Digital copper | Internet (IP) |
| Infrastructure | Physical wire to your premises | Physical wire + digital channels | Your broadband connection |
| Concurrent calls | 1 per line | 2 (BRI) or 30 (PRI) per connection | Unlimited (bandwidth dependent) |
| Scalability | Engineer visit required | Engineer visit required | Instant, from a dashboard |
| Location-dependent | Yes | Yes | No |
| Status in Ireland | Being phased out | Being phased out | Current standard |
| Cost | Per line rental | Per channel rental | Per user or per trunk |
| Mobile working | ✗ | ✗ | ✓ |
| Features | Basic | Moderate | Full UCaaS capability |
Why SIP Matters for Irish Businesses Right Now
The shift from PSTN and ISDN to SIP-based telephony isn’t a future trend — it’s happening now, with a hard deadline attached for Northern Irish businesses and a clear direction of travel for the Republic.
More importantly, SIP isn’t just a like-for-like replacement for copper lines. It’s the foundation for a fundamentally more capable phone system:
Mobile working. SIP-based systems work on any device with an internet connection. Your team makes and receives business calls on their existing mobiles and laptops — from the office, from home, or from a site in Connaught.
Intelligent routing. SIP systems route calls dynamically based on rules you define — time of day, department, availability, caller identity. This isn’t possible on a basic PSTN or ISDN line.
AI call handling. Niamh, Dromlik’s AI receptionist, answers calls when your team can’t — taking bookings, answering queries, capturing leads, and routing urgent calls to the right person. This capability is built on SIP infrastructure.
CRM integration. When a call comes in, the caller’s record opens automatically in HubSpot, Salesforce, Zoho, or Zendesk. After the call, it logs automatically. This requires the call to be processed through an IP-based system — not possible with a copper PSTN or ISDN line.
No hardware to maintain. A SIP-based cloud system has no on-site hardware beyond the devices your team already carries. No PBX cabinet, no engineer visits, no hardware refresh cycles.
SIP Trunking vs a Cloud Phone System: What’s the Difference?
This distinction confuses a lot of Irish businesses. Both use SIP technology — but they serve different purposes:
SIP trunking connects an existing on-premise PBX to the telephone network via the internet, replacing physical ISDN or PSTN lines. The PBX hardware stays on-site; only the line connection changes. It’s a migration step for businesses with a working PBX that isn’t ready to replace.
A cloud phone system (like Dromlik) replaces the on-premise PBX entirely. The entire system — routing, IVR, voicemail, recording, extensions — lives in the cloud. Your team uses apps on their existing devices. There’s no hardware to manage.
For most Irish businesses evaluating this in 2026, a cloud phone system is the right destination — SIP trunking is a transitional step that makes sense if you have a recent, well-functioning PBX with years of useful life remaining. If the PBX is aging, skipping straight to a cloud system is typically more cost-effective in total.
You don’t need to understand SIP to use Dromlik — just like you don’t need to understand TCP/IP to use the internet. Dromlik handles the SIP infrastructure. You get Irish local numbers, call routing, mobile apps, and an AI receptionist, managed for you.
How Dromlik Uses SIP for Irish Businesses
Dromlik’s cloud phone system is built on SIP infrastructure — but you never have to configure or manage SIP directly. The Dromlik team handles:
- Irish local numbers across 15 area codes, provisioned instantly
- SIP-based call routing configured to your specifications
- Number porting from existing PSTN or ISDN lines, free of charge
- Integration with your CRM, your booking system, and Microsoft Teams
- AI receptionist (Niamh) answering calls in natural Irish English
The underlying technology is SIP. What you experience is a professional Irish business phone system that works on your team’s existing devices, configured and managed by Dromlik.
Moving From ISDN to Dromlik
If your business is currently on an ISDN line — whether ISDN2 or ISDN30 — migrating to Dromlik is the right move:
- Your existing Irish number ports across — free, managed by Dromlik, 5–10 business days, no gap in service
- Dromlik configures your system — routing rules, IVR, greetings, integrations, AI receptionist
- Your team downloads the app — calls live on existing mobiles and laptops
- Your ISDN line is cancelled — you’re no longer paying ISDN line rental
The result: a significantly more capable phone system at a lower monthly cost than ISDN line rental, with no hardware to manage.
Pricing
| Plan | Monthly | Annual | What’s Included |
|---|---|---|---|
| Nucleus | €15/user/month | €12/user/month | Calling, IVR, routing, call recording, voicemail to email, mobile and desktop apps, Teams integration |
| Orbit | €27/user/month | €22/user/month | Everything in Nucleus plus omnichannel, CRM integrations, call centre queues, AI transcription |
| AI Receptionist (Niamh) | +€100/month system-wide | +€100/month system-wide | 24/7 AI call answering, 200 minutes included |
Irish local numbers from €5/month. Free number porting from ISDN or PSTN. No setup fees. 7-day free trial.
See the full pricing breakdown.
Replace your ISDN line with a modern cloud phone system
Talk to the Dromlik team about migrating from ISDN or PSTN to a fully managed cloud phone system. Free 7-day trial, no setup fees, free number porting.
