SIP Trunking in Plain English

SIP stands for Session Initiation Protocol. In simple terms, a SIP trunk is a virtual phone line that connects your business phone system to the outside world using your internet connection instead of physical copper wires.

Think of it this way: traditional phone lines are like dedicated roads between your office and the phone network. SIP trunks use the internet highway instead — it's faster, cheaper, and more flexible.

How Does SIP Trunking Work?

Here's the basic flow:

• Your phone system (PBX) connects to a SIP trunk provider over your internet connection
• When someone calls your business number, the call arrives via the SIP trunk
• When your staff make outbound calls, they go out via the SIP trunk
• The SIP provider handles the connection between the internet and the traditional phone network

Each SIP trunk channel supports one concurrent call. So if you need 10 people to be on the phone simultaneously, you need 10 channels. Unlike ISDN where you buy lines in blocks of 2 or 30, SIP trunks let you buy exactly the number of channels you need.

SIP Trunking vs ISDN

SIP trunking is the direct replacement for ISDN lines, which are being switched off with the PSTN by 2027. Here's how they compare:

• Cost: SIP trunks cost £5-10 per channel/month vs £15-25 per ISDN channel
• Flexibility: Add or remove channels instantly vs waiting for engineer visits
• Features: SIP supports HD voice, video, and advanced routing. ISDN is voice-only
• Scalability: Buy exactly the channels you need vs fixed blocks of 2 or 30
• Resilience: SIP can failover to backup connections. ISDN has a single point of failure

SIP Trunking vs Hosted VoIP

This is an important distinction. Hosted VoIP is a complete phone system in the cloud — you don't need any on-premise PBX hardware. SIP trunking connects an existing on-premise PBX to the phone network.

• Choose SIP trunking if: you have an existing IP-PBX system you want to keep using
• Choose hosted VoIP if: you want a complete cloud phone system with no on-premise hardware
• Choose hosted VoIP if: you're a small business without IT staff to manage a PBX

For most small and medium businesses, hosted VoIP is the simpler and more cost-effective option. SIP trunking makes sense for larger organisations with existing PBX investments.

Benefits of SIP Trunking

Significant Cost Savings

Businesses typically save 40-60% on their phone line costs by switching from ISDN to SIP. A business with 30 ISDN channels paying £600/month could reduce that to £200-300/month with SIP trunks.

Instant Scalability

Need more capacity for a busy period? Add channels in minutes through your provider's portal. Quiet period? Scale back down. No engineer visits, no waiting.

Business Continuity

SIP trunks can be configured to failover to alternative locations, mobile phones, or backup internet connections. If your office broadband goes down, calls can automatically route to your 4G/5G backup connection or to mobile phones.

Number Portability

Keep all your existing business phone numbers when switching to SIP. Your customers won't notice any change — except perhaps better call quality.

Geographic Flexibility

With SIP, you can have phone numbers from any UK area code, regardless of where your office is physically located. A Belfast business could have London, Dublin, or Manchester numbers for local presence.

What You Need for SIP Trunking

• Compatible PBX: Your phone system must support SIP. Most modern IP-PBX systems do (Avaya, Panasonic, Cisco, 3CX, Asterisk)
• Reliable broadband: Each concurrent call uses about 100kbps. 10 simultaneous calls need about 1Mbps of dedicated bandwidth
• Quality of Service (QoS): Your router should prioritise voice traffic over data
• A reputable SIP provider: Choose a provider with UK-based support and proven reliability

SIP Trunking Costs Breakdown

• Per channel: £5-10 per month
• Call charges: Often included or 1-2p per minute for UK calls
• Number porting: Usually free
• Setup: Free to £100 depending on provider
• No hardware required — connects to your existing PBX

Common SIP Trunking Questions

Can I keep my phone numbers?

Yes. Number porting is standard practice. All your existing numbers transfer to SIP with no disruption to callers.

Is SIP trunking reliable?

With a good broadband connection and QoS configuration, SIP trunking is highly reliable. Add a backup internet connection for near-100% uptime.

How many channels do I need?

A common rule of thumb is one channel per 3-4 users. So a 20-person office typically needs 5-7 SIP channels. Your provider can analyse your call patterns to recommend the right number.