If you’ve been told you need “SIP trunks” for your phone system and have no idea what that means, this guide is for you. No jargon. No acronym overload. Just a clear explanation of what SIP trunking is, how it works, what it costs, and whether your business should be using it.
The Simple Explanation
A SIP trunk is a virtual phone line that runs over the internet instead of a physical copper wire.
Traditional phone lines (ISDN, PSTN) use physical cables between your office and the telephone exchange. Each line can carry one call. If you need 10 simultaneous calls, you need 10 lines — each with a monthly rental fee.
SIP trunks replace those physical lines with internet connections. Your phone system connects to a SIP trunk provider over your existing broadband, and calls flow over the internet. No copper. No physical lines. No line rental per channel.
SIP stands for Session Initiation Protocol — it’s the technology that sets up, manages, and ends voice calls over the internet.
Trunk is a telecom term for a connection that carries multiple calls. A SIP trunk can carry as many simultaneous calls as your internet bandwidth allows.
How Does SIP Trunking Work?
Here’s the flow when someone calls you:
- The caller dials your business number (which looks like a normal phone number)
- The call reaches your SIP trunk provider’s network
- The provider routes the call over the internet to your phone system (3CX, Yeastar, etc.)
- Your phone system rings the right extension, queue, or IVR
And when you make an outbound call:
- You dial the number on your desk phone, mobile app, or softphone
- Your phone system sends the call to your SIP trunk provider
- The provider connects the call to the destination (landline, mobile, international)
- The call is established
From the user’s perspective, nothing changes. Calls sound the same. Numbers look the same. The difference is behind the scenes — instead of physical copper lines, calls travel over the internet.
Why Are Businesses Switching to SIP Trunks?
1. ISDN Is Dead
ISDN (Integrated Services Digital Network) — the technology behind traditional business phone lines — is being switched off across Europe and the world. In the UK, the PSTN shutdown is scheduled for 2027. Ireland, Germany, France, and Australia have already started decommissioning ISDN. If your business still runs on ISDN lines, you need to migrate to SIP before the shutdown.
2. Cost Savings of 40-60%
SIP trunks are dramatically cheaper than ISDN:
| Cost | ISDN (per channel) | SIP Trunk |
|---|---|---|
| Line rental | €15-30/month per channel | €3-8/month per channel |
| Local calls | €0.02-0.05/min | €0.01-0.02/min (or bundled) |
| International calls | €0.10-0.50/min | €0.02-0.10/min |
| Setup fee | €50-200 | Usually free |
| Hardware | PRI/BRI cards required | No additional hardware |
A business with 10 ISDN channels paying €25/channel = €250/month in line rental alone. The same capacity via SIP trunking might cost €50-80/month. That’s €2,000+/year saved — just on line rental, before call charges.
3. Flexibility
- Scale instantly — need more capacity? Add channels in minutes, not weeks
- Work from anywhere — SIP trunks connect to cloud phone systems, so your team can make/receive calls from home, mobile, or any office
- Number portability — keep your existing numbers when switching
- Geographic freedom — get phone numbers in any country/city without a physical presence
4. Features
SIP trunks enable features that ISDN can’t:
- HD voice quality (wideband codecs like G.722 and Opus)
- Easy failover and redundancy
- Call recording at the network level
- CLI (caller ID) flexibility — present different numbers for different departments
- SMS capability on some providers
What Do You Need for SIP Trunking?
- An internet connection — minimum 100kbps per concurrent call. A standard 100Mbps broadband can handle 100+ simultaneous calls. Dedicated/leased lines are recommended for larger deployments.
- A phone system (PBX) — this is the software that manages your calls, extensions, IVR, queues, etc. Options include:
- A SIP trunk provider — this is the company that provides the SIP trunks and connects your phone system to the phone network. This is where ITelecoms comes in.
- IP phones or softphones — desk phones that connect via ethernet (Yealink, Fanvil, Grandstream, Poly) or software clients on your computer/mobile.
How to Choose a SIP Trunk Provider
Not all SIP trunk providers are equal. Here’s what to look for:
Call Quality
The provider’s network infrastructure directly affects call quality. Look for:
- Multiple Points of Presence (PoPs) geographically close to you
- Redundant network paths
- Support for HD codecs (G.722, Opus)
- Low latency and jitter
Pricing Transparency
Watch out for:
- Hidden setup fees
- Per-minute charges vs bundled minutes
- Connection fees (charged per call on top of per-minute)
- Minimum contract terms and exit fees
Number Porting
Confirm the provider can port your existing numbers. This should be included free or at minimal cost.
Reliability
- Uptime SLA (look for 99.9%+)
- Redundancy and failover
- 24/7 monitoring
Support
When your phone system goes down, you need someone who answers immediately — not a ticketing system.
SIP Trunking vs VoIP: What’s the Difference?
People often use “SIP trunking” and “VoIP” interchangeably, but they’re not the same thing.
VoIP (Voice over Internet Protocol) is the broad technology of making voice calls over the internet. It includes everything from WhatsApp calls to Zoom to business phone systems.
SIP trunking is a specific type of VoIP connection that replaces traditional phone lines. It connects your PBX (phone system) to the phone network. It’s the plumbing, not the faucet.
Think of it this way:
- VoIP = making calls over the internet (general concept)
- SIP trunk = the specific connection between your phone system and the phone network (specific implementation)
All SIP trunking is VoIP, but not all VoIP is SIP trunking.
Common Concerns
“Will call quality be as good as ISDN?”
With a decent internet connection and a good SIP provider, yes — call quality is equal or better. HD voice codecs on SIP deliver clearer audio than ISDN’s G.711 codec. The key is having sufficient bandwidth and a reliable connection.
“What happens if the internet goes down?”
Most SIP trunk providers offer failover options:
- Redirect calls to a mobile number
- Route to a secondary internet connection
- Queue calls and deliver voicemail
- Failover to a backup SIP trunk on a different connection
A 4G/5G backup broadband connection solves this for most businesses.
“Is SIP trunking secure?”
SIP trunks support TLS (encrypted signalling) and SRTP (encrypted audio). When properly configured, calls are encrypted end-to-end. Your SIP trunk provider should offer these as standard.
“Can I keep my phone numbers?”
Yes. Number porting is standard practice. Your existing numbers are transferred from your old provider to your SIP trunk provider. The process typically takes 5-10 working days.
Frequently Asked Questions
How many SIP trunks do I need?
A rule of thumb: you need one SIP channel per simultaneous call. A 20-person office typically needs 5-8 channels (not everyone calls at the same time). Your provider can help you size this based on your call patterns.
Can SIP trunks work with my existing phone system?
If your PBX supports SIP (most modern systems do), yes. If you’re on a very old ISDN-only system, you’ll need to upgrade or add a SIP gateway. This is a good time to move to 3CX or Yeastar anyway.
Do I need a static IP address?
Most SIP providers prefer a static IP for registration and security, but some support dynamic IP with re-registration. A static IP is recommended for reliability.
What’s the difference between SIP trunking and hosted VoIP?
- SIP trunking = you have your own phone system (PBX) and the SIP trunk connects it to the phone network
- Hosted VoIP = the phone system is managed by the provider, you just plug in phones
SIP trunking gives you more control. Hosted VoIP is simpler but less flexible.
Get Started with SIP Trunking
ITelecoms provides SIP trunking with:
- Competitive call rates (domestic and international)
- Number porting from any provider
- HD voice quality
- 99.9%+ uptime SLA
- 24/7 support
- Works with 3CX, Yeastar, Teams, and any SIP-compatible system
Get a SIP trunking quote or learn more about our SIP trunking services.
Still on ISDN? The clock is ticking. Contact ITelecoms to plan your migration to SIP trunking before the ISDN shutdown.