SIP Line vs. SIP Trunk and How Many of Each You Need

Session Initiation Protocol (SIP) is the technical side of what starts, maintains, and terminates VoIP voice and video calls. Ever since the Internet Engineering Task Force (IETF) standardized SIP in 1999, SIP has been the framework for communication sessions through VoIP.

The terms SIP line and SIP trunk both refer to communication systems that use VoIP to support voice and video calls over the internet, but they’re two slightly different things.

A SIP line is an individual connection associated with a single extension or user that typically carries calls over a SIP-based VoIP network.

Meanwhile, a SIP trunk is a virtual connection that can carry multiple communication channels between a company’s private branch exchange (PBX) and its service provider’s network. This connection allows companies to make and receive calls on multiple SIP lines via the internet instead of the public switched telephone network (PSTN).

As a result, most businesses don’t need more than one SIP trunk because a single trunk can support many lines.

The Technical Side of SIP Lines vs. SIP Trunks

SIP Lines

• From a user perspective, a SIP line is much like a traditional telephone line, only it uses SIP and the internet instead of the PSTN.
• Like an old-fashioned phone line, a SIP line represents one communication channel. In general, VoIP systems are more flexible than the PSTN, and the one-line-per-call framework simulates older phone systems for consistency and simplicity.
• Although SIP technology is virtual, it borrows the term “line” from traditional telephony to help users conceptualize how it works. As such, there’s no physical line that carries communications like in conventional telephony.
• Each SIP line usually represents a single user or extension. If a company wants to engage multiple calls simultaneously, they need an individual SIP line for each call. To support multiple SIP lines, a company needs a SIP trunk.

SIP Trunks

• SIP trunks represent multiple virtual lines bundled together; they allow businesses to connect their phone systems (typically a PBX) to the internet for making calls.
• The term “trunk” in traditional telephony refers to a high-capacity line that connects telephone exchanges. It has similarly been carried over from the traditional system for consistency.
• A SIP trunk can handle multiple calls simultaneously. For this reason, SIP trunks make sense for companies that handle a high volume of calls.
• Instead of a single line of communication that’s restricted to one user, a SIP trunk offers a pool of lines agents can share across an entire organization.

You can configure a SIP trunk to handle a set number of channels. Each channel can take a concurrent call—inbound or outbound. For example, a SIP trunk with twenty channels can handle twenty calls at once.

The differences between inbound and outbound lines are straightforward.

• Inbound SIP Lines: These are lines that receive incoming voice and video calls from an external source. When someone calls your business, an inbound SIP line handles the connection, routing the call to the appropriate extension.

• Outbound SIP Lines: 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 simultaneously active communications (including both voice and video connections) count as concurrent calls. Thus, if five employees are engaged in active phone conversations—that’s five concurrent calls.

The number of concurrent calls a SIP system can handle depends on several factors. For instance, the overall availability of bandwidth sets a hard limit on the number of calls a system can support. That said, some codecs compress audio more efficiently than others, making better use of available bandwidth.

The capacity of your SIP trunk is another crucial factor for concurrent call capacity. Each service provider and plan uses a different infrastructure, which can impact the number of available lines. Equipment like your Session Border Controller (SBC) and PBX will also affect the concurrent call capacity before maxing out.

There are several factors to consider when assessing the number of SIP channels your company needs. Call volume and the number of employees are the most crucial of these factors.

If you can predict the approximate call volumes at peak times, that’ll help you determine how many lines you require.

When you’re ready to purchase a plan, you should also consider the idea of future expansion. For instance, if your company is likely to snowball or experience large seasonal fluctuations, you may want to consider a SIP trunking plan that allows you to expand the number of lines very quickly and affordably.

Budgetary concerns also play a role in selecting the right SIP system. Typically, the more lines a plan supports, the higher it costs per month. You’ll want to shop around with SIP trunking service providers to see what each one can offer your company according to your needs and potential for growth.

Situations for multiple SIP trunks?

Most companies only need a single trunk because one SIP trunk can support multiple SIP lines. Small to medium-sized businesses (SMBs) can purchase plans that offer 10-30 channels from a single trunk.

SIP trunking services offer plans for larger businesses, too. There are plans with the capacity for over 100 channels on a single SIP trunk. However, there are also cases where having multiple trunks can be more beneficial.

Large companies that handle many calls can often benefit from multiple SIP trunks. Since each trunk comes with a set number of lines, companies that use several trunks can dramatically increase the capacity of their communications systems.

Having multiple SIP trunks also allows for load balancing. This means calls can be spread across multiple trunks, preventing any single trunk from overloading. Load balancing can be a critical measure for some companies to maintain stable and efficient communications.

Another reason why a company may use several SIP trunks is if it has multiple bases spread across many geographic locations. In these cases, call traffic routes to a specific trunk based on the caller’s location. This process reduces latency and ensures better call quality.

Similarly, having a backup SIP trunk can also be useful for minimizing service disruptions. When one trunk experiences issues or is down for maintenance, for example, calls will redirect to the functional trunk.

Using different service providers for each trunk further protects against disruption—because if one company is experiencing problems, you can switch to the other as a failover.

Confusion Surrounding SIP Line vs. SIP Trunk

The technical difference between SIP lines and trunks is straightforward—lines represent single channels associated with one user or extensions, and trunks refer to bundles of channels.

Unfortunately, “SIP trunk” and “SIP line” are sometimes used interchangeably (typically by the general public, not by those in the VoIP or SIP industry). This leads to confusion and a lot of misconceptions.

For example, there’s a common misconception that you need a SIP trunk for every concurrent call. In reality, you need a SIP line for every concurrent call.

In telecommunications terms, a “trunk” has always carried multiple lines, which is why SIP service providers offer trunk packages that support a set number of simultaneous calls.

Meanwhile, many people remain convinced they need multiple trunks because of a misunderstanding of the terminology. Just remember that VoIP and other IT professionals unanimously agree that a trunk can always support multiple lines.

Other Names for SIP Line and SIP Trunk

The telecommunication industry has many overlapping terms with nuanced differences in meaning. From a user point of view, we can simplify things by grouping similar terms.

Similar terms to SIP Line

• SIP Channel: SIP channel and SIP line both represent the capacity to carry a single voice call. For users, there’s no practical difference between the terms.
• Voice Channel: The term “voice channel” is used in several types of communication technology, including traditional telephony. In SIP and VoIP technology, voice channels and SIP lines broadly mean the same thing.
• Call Path: Call path refers to a voice call’s entire journey through routers, switches, and gateways. It’s part of a SIP line—the part that maintains the connection. But call path and SIP line aren’t interchangeable terms. Call path refers to the route, not just the channel.

Similar terms to SIP Trunk

• VoIP Trunk: A VoIP trunk is a general term encompassing several different technologies for transmitting voice over the internet, including SIP. A SIP trunk is a specific infrastructure that connects a PBX to an ITSP or VoIP service provider.
• Voice Trunk: Similar to a voice channel, the term voice trunk appears in several types of communication technology, including traditional telephony. It refers to the pathway that carries voice signals. A SIP trunk is a kind of voice trunk.
• IP Trunk: An IP trunk is a communication link for transmitting both voice and other data through the internet. An IP trunk can use several types of signaling protocols, including SIP.