SIP line and SIP trunk are both VoIP-based communication systems for handling voice and video calls, but they serve different purposes. A SIP line is a dedicated connection tied to a single extension or user, typically carrying calls over a SIP network. In contrast, a SIP trunk is a virtual connection that aggregates multiple communication channels between a company's PBX and its service provider. This setup enables businesses to manage numerous SIP lines via a single trunk rather than relying on the traditional PSTN. Consequently, most organizations find that one SIP trunk is sufficient to support all their communication needs.
The Technical Side of SIP Lines vs. SIP Trunks
SIP Lines
From a user's perspective, a
SIP line functions similarly to a traditional telephone line but operates over the internet using the SIP protocol instead of the PSTN.
Like conventional phone lines, a SIP line represents a single communication channel. While VoIP systems offer greater flexibility than PSTN, the one-line-per-call structure mimics older phone systems for familiarity and ease of use.
Despite being a virtual technology, SIP adopts the term "line" from traditional telephony to help users better understand its function. Unlike conventional systems, there is no physical line transmitting communications.
Each SIP line typically corresponds to a single user or extension. To handle multiple calls at the same time, a business needs a separate SIP line for each concurrent call. To support multiple SIP lines efficiently, a company requires a SIP trunk.
SIP Trunks
SIP trunks are virtual connections that bundle multiple SIP lines, enabling businesses to link their phone systems (typically a PBX) to the internet for voice communication.
In traditional telephony, the term “trunk” refers to a high-capacity line that connects telephone exchanges. This concept has been retained in SIP technology for consistency and ease of understanding. Some trunks are metered while others offer unlimited outbound calls. It depends on the providers’ business model and pricing plans.
A SIP trunk can support multiple concurrent calls, making it an ideal solution for businesses that manage high call volumes. Unlike a single SIP line, which is limited to one user per call, a SIP trunk provides a shared pool of lines that can be accessed across an entire organization.
SIP trunks can be configured with a specific number of channels, with each channel capable of handling one inbound or outbound call. For example, a SIP trunk with twenty channels can accommodate twenty simultaneous calls.
The distinction between inbound and outbound SIP lines is simple and clear.
Inbound SIP Lines: These lines are dedicated to receiving incoming voice and video calls from external sources. When someone dials your business, an inbound SIP line manages the connection, directing the call to the appropriate extension.
Outbound SIP Lines: An outbound SIP line is a connection that allows a business to make calls to external parties. SIP stands for Session Initiation Protocol, which is a technology that manages VoIP sessions. These lines initiate calls and video calls to external sources. When someone in your company calls a number, outbound SIP lines establish the connection to the external party.
What Counts as a Concurrent Call?
Any active communication, including voice and video connections, counts as a concurrent call. For example, if five employees are on active phone calls, that's five concurrent calls.
The number of concurrent calls a
SIP system can handle depends on factors like available bandwidth, which sets a limit on the number of calls a system can support. Some codecs compress audio more efficiently, optimizing bandwidth usage.
The SIP trunk capacity also affects concurrent call volume, as service providers and plans use different infrastructures that impact available lines. Equipment such as the Session Border Controller (SBC) and PBX also play a role in determining the concurrent call capacity.
When assessing your company's needs for SIP channels, call volume and the number of employees are key factors. Predicting peak call volumes can help determine how many lines are needed.
Consider future expansion when selecting a SIP plan, especially if your company is expected to grow or experience seasonal fluctuations. Choose a
SIP trunking plan that allows for easy and cost-effective line expansion.
Budget is also a consideration, as plans with more lines tend to cost more. Comparing SIP trunking service providers will help you find the best plan for your needs and potential growth.
Confusion Surrounding SIP Line vs. SIP Trunk
The technical difference between SIP lines and trunks is simple—lines are individual channels for one user or extension, while trunks are bundles of channels.
However, "SIP trunk" and "SIP line" are often used interchangeably, particularly by the general public, leading to confusion and misconceptions.
A common misconception is that you need a SIP trunk for each concurrent call. In reality, you need a SIP line for each concurrent call.
In telecommunications, a "trunk" has always carried multiple lines, which is why SIP service providers offer trunk packages that support a specific number of simultaneous calls.
Despite this, many still believe they need multiple trunks due to terminology confusion. It's important to note that VoIP and IT professionals agree that a trunk can support multiple lines.
| |
SIP line |
SIP trunk |
| What it connects |
A single device to the internet |
A company's phone system to the internet |
| How many calls it can handle |
One call at a time |
Multiple calls simultaneously |
| Who it's best for |
Small businesses with low call volume |
Larger businesses or those with fluctuating call volumes |
SIP Line vs. SIP Trunk
SIP trunks and SIP lines are often confused, though they can be interchangeable in some contexts.
A SIP line connects a single device for making VoIP calls over the internet, enabling that device to make and receive calls.
A SIP trunk connects an entire phone system or PBX to a VoIP provider, allowing the entire internal phone network to make and receive calls.
Capacity - SIP Line: One device can make/receive calls.
- SIP Trunk: Multiple devices/phones can make calls simultaneously. A SIP trunk is one-to-many, while a SIP line is one-to-one.
Features - SIP Line: Basic call capabilities for a single device.
- SIP Trunk: Enables features like call routing, IVRs, and call handling/transferring via the business PBX, with intelligence on the phone system rather than the end device.
Scalability - SIP Line: Adding lines is labor-intensive, as each device needs its own line.
- SIP Trunk: Easy to scale by adding more phones to the existing PBX/SIP trunk as the business grows.
