SIP Trunking Connect Your PBX to the Cloud
SIP trunking connects a compatible business PBX to the public telephone network over an IP connection, replacing or reducing reliance on traditional ISDN or other physical phone lines. Businesses with a SIP-compatible PBX may be able to keep much of their existing phone-system infrastructure while gaining more flexible calling capacity.
Connect Your Existing PBX to VoIP
SIP trunking allows a compatible on-premises PBX to make and receive external calls through an IP-based service provider. This can help businesses modernize their calling infrastructure without immediately replacing the entire PBX system.
What is SIP Trunking?
SIP trunking is a VoIP technology that connects a business PBX to a telephone service provider over an IP network. Instead of relying on traditional physical phone lines, a SIP trunk carries call signaling and supports multiple simultaneous calls through the business’s data connection.
How Does SIP Trunking Work?
When an employee makes an external call, the PBX sends SIP signaling to the SIP trunk provider over the business’s IP connection. The provider then routes the call toward the destination, which may be another VoIP user, mobile phone or traditional telephone number.
For incoming calls, the provider receives the call and sends it through the SIP trunk to the company’s PBX. The PBX then applies its configured call-routing rules and sends the call to an extension, ring group, IVR, queue or other destination.
SIP manages the signaling needed to establish and end the session, while protocols such as RTP typically carry the actual voice media.
SIP trunking call-flow diagram
Business Phones
↓
PBX
↓
SIP Trunk
↓
SIP Provider
↓
PSTN / Mobile / Other VoIP Networks
Sip Trunking Benefits
Reduce costs with SIP Trunking
SIP trunking can reduce reliance on separate physical telephone circuits and may lower calling or connectivity costs, particularly for businesses with multiple locations or significant call volumes. Actual savings depend on the provider, call patterns and existing infrastructure.
Scalable Call Capacity
Businesses can often increase or decrease their simultaneous calling capacity more easily than with traditional physical telephone circuits. This makes SIP trunking useful for growing teams and organizations with changing call volumes.
Connect Multiple Business Locations
SIP-based connectivity can help organizations connect calling infrastructure across multiple offices and simplify communication between locations, depending on the PBX and provider configuration.
Add Business Phone Numbers
SIP trunk providers can typically supply additional direct-dial numbers (DIDs/DDIs), allowing businesses to assign numbers to departments, employees or locations without installing a separate physical line for each number.
Faster Provisioning
Because SIP trunks are software-based services rather than individual physical telephone lines, additional calling capacity can often be provisioned more quickly. Provisioning times vary by provider, number requirements and network configuration.
Re-routing option available
Depending on the provider, incoming calls can be redirected to alternative numbers, locations or systems if the primary office or PBX becomes unavailable. This can form part of a broader business-continuity and disaster-recovery plan.
Works With PBX Calling Features
A SIP trunk provides external calling connectivity, while a compatible PBX can continue to provide features such as IVR, call queues, ring groups, voicemail and advanced call routing. Available features depend on the PBX and service provider.
Flexible Call Routing
Businesses can configure their PBX or provider routing to send unanswered or unavailable calls to another extension, mobile number, voicemail box or backup destination.
Keep Existing Business Numbers Where Porting Is Supported
Businesses may be able to retain existing telephone numbers when moving to a SIP trunk provider through number porting. Availability, eligibility, costs and transfer times depend on the provider, country and number type.
SIP Trunk vs SIP Channel
A SIP trunk is the virtual connection between a PBX and SIP service provider, while a SIP channel generally represents the capacity for one simultaneous call. For example, a business may have one SIP trunk configured with enough channels to support multiple concurrent inbound and outbound calls. Terminology and capacity models can vary by provider.
SIP Trunking vs VoIP
| SIP Trunking | VoIP | |
|---|---|---|
| What it is | PBX connectivity/service using IP | Broad technology for voice over IP |
| Main purpose | Connect PBX to external calling network/provider | Carry voice communications over IP |
| Existing PBX | Often used with compatible PBXs | May or may not require a PBX |
| Business use | Common for organizations retaining PBX infrastructure | Used in everything from apps to cloud phone systems |
SIP Trunking vs PRI
| Feature | SIP Trunk | PRI |
|---|---|---|
| Connection | IP-based | Digital circuit |
| Capacity | Often flexible | Traditionally fixed channel structure |
| Scaling | Usually easier | Additional circuit/capacity may be needed |
| Infrastructure | Data/IP network | Dedicated telecom circuit |
| PBX requirement | SIP-compatible or gateway | PRI-compatible |
| Disaster routing | Often flexible | More dependent on carrier/configuration |
SIP trunking requirements
- compatible PBX or SIP gateway;
- SIP trunk provider;
- reliable IP connectivity;
- sufficient bandwidth;
- properly configured router/firewall;
- QoS where appropriate;
- compatible codecs;
- phone numbers/DIDs;
- security configuration;
- possibly SBC depending on deployment.
Is SIP Trunking Secure?
Yes, SIP trunking can be secure when it is properly configured and protected. Businesses can reduce risks by using strong authentication, firewalls or Session Border Controllers (SBCs), restricting unauthorized SIP traffic, and keeping PBX software up to date. Some providers also support TLS encryption for SIP signaling and SRTP for voice traffic. However, security depends on the SIP provider, PBX configuration, network setup, and the security measures being used.