SIP trunking is a method of delivering telephone services over the internet to an existing on-premise PBX (Private Branch Exchange) phone system.
SIP stands for Session Initiation Protocol — it's the technology that establishes, manages, and terminates real-time communication sessions (voice calls, video calls, messaging) over the internet.
Trunking is a telecommunications term for a shared communication line. Traditional phone systems used physical "trunk lines" (bundles of copper wires) to connect to the telephone network. SIP trunking replaces those physical lines with internet connections.
In simple terms: SIP trunking lets your old phone system make calls over the internet instead of over expensive copper lines.
How Does SIP Trunking Work?
- Your business has an existing PBX (IP-PBX or traditional PBX with an adapter)
- A SIP trunking provider (like CallOrbit) connects your PBX to the public switched telephone network (PSTN) via the internet
- When a call is made, the PBX sends the call data through the SIP trunk to the provider
- The provider routes the call to its destination (landline, mobile, or another VoIP system)
[Your PBX] → [Internet/SIP Trunk] → [SIP Provider] → [PSTN/Destination]SIP Trunking vs. VoIP vs. Cloud Phone System
| Feature | SIP Trunking | VoIP (General) | Cloud Phone System |
|---|---|---|---|
| What it does | Connects existing PBX to internet | Transmits voice over internet | Fully hosted phone system |
| Hardware required | Existing PBX | Minimal (softphone/IP phone) | None |
| Management | You manage PBX | Provider manages | Provider manages everything |
| Best for | Companies with existing PBX investment | General business use | Companies wanting no hardware |
| Upfront cost | Low (keeps existing PBX) | Low | Lowest |
| Ongoing cost | Low (per-channel/per-minute) | Low (subscription) | Low (subscription) |
| Scalability | Moderate | High | Highest |
The relationship: VoIP is the underlying technology. SIP trunking is a way to use VoIP with an existing PBX. A cloud phone system is a fully hosted VoIP solution that replaces the PBX entirely.
Benefits of SIP Trunking
1. Cost Reduction
SIP trunks are 40–60% cheaper than traditional ISDN or PRI trunk lines. You eliminate per-line rental fees and reduce long-distance charges.
2. Leverage Existing Investment
If your business spent $10,000+ on a PBX system, SIP trunking lets you keep using that hardware while modernizing your connectivity.
3. Scalability
Adding a SIP channel takes minutes, not the weeks required to install a new physical trunk line.
4. Geographic Flexibility
Get virtual phone numbers in any area code — route them through your PBX. Have a 647 (Toronto) number and a 310 (LA) number ringing the same office system.
5. Business Continuity
If your office is unreachable, calls can be rerouted to mobile phones or alternate locations through your SIP trunk provider.
When to Use SIP Trunking vs. a Cloud Phone System
Choose SIP Trunking If:
- You have a significant investment in on-premise PBX hardware that's still functional
- Your IT team can manage and maintain the PBX
- You need granular control over your call routing and configuration
- You're in an industry with specific compliance requirements that mandate on-premise control
Choose a Cloud Phone System If:
- You're a startup or small business without existing PBX hardware
- You want zero maintenance responsibility
- You need a system that works for remote teams
- You want to set up a phone system in minutes, not days
- You're planning to scale quickly
For most small and medium businesses, a cloud phone system is the better choice. SIP trunking is ideal for larger organizations with existing infrastructure.
How to Set Up SIP Trunking
- Ensure your PBX is SIP-compatible (IP-PBX, or traditional PBX with an ATA/gateway)
- Choose a SIP trunking provider (CallOrbit offers SIP trunking services)
- Configure your PBX with the provider's SIP credentials
- Port your existing numbers (number porting guide) or get new ones
- Test call quality before you go live
- Go live
Frequently Asked Questions
- How many SIP trunks do I need?
- Each SIP trunk (channel) supports one concurrent call. If you expect a maximum of 10 simultaneous calls, you need 10 channels. A rule of thumb: 1 channel per 3–5 employees.
- Is SIP trunking secure?
- SIP trunking can be secured with TLS (signaling encryption) and SRTP (media encryption) when it is configured and monitored correctly.
- Can I use SIP trunking with my existing phone numbers?
- Yes. Port your numbers to your SIP trunking provider.
- What internet speed do I need?
- Approximately 100 kbps per concurrent call. For 10 concurrent calls, you'd want at least 1 Mbps dedicated to voice traffic, plus bandwidth for data.
Get Started with SIP Trunking
CallOrbit offers SIP trunking for businesses with existing PBX systems — plus full cloud phone system solutions for those ready to go fully cloud.
Talk to Our Team About SIP Trunking →