What IsSIP Trunking?

Session Initial Protocol Trunking, also known as SIP trunking is the process of sending and receiving voice or video calls via the internet. It is similar to a traditional phone system minus the cables or physical network connections—virtual SIP lines have replaced them all.

Physical wires made it possible to have long-distance telephone communication. As telephone communication progressed, wired networks multiplied in quantity. But, the wires had two major problems.

  1. The cables—especially longer wires—were prone to physical damage in case of disasters or emergencies; fixing them was not an easy task
  2. The wires were expensive to buy and maintain, which was unprofitable for businesses

Long-distance phone calls in numerous business domains are more urgent than ever. With SIP trunking, we can now use the internet to transfer voice and video messages for longer distances—it significantly cuts down the cost of phone calls.

For a better understanding of call trunking, let's elaborate on SIP, PBX, and Trunk, and what their functions are.

What Is SIP?

Session Initiation Protocol (SIP) is an application layer protocol that uses an internet connection to transfer voice or video messages from one point to another. It replaces traditional wires with virtual channels that allow two ends of PBX (Private Branch Exchange Phone System) to connect. Simply put, SIP is the technology that creates, modifies, and terminates sessions with one or more parties in an IP network, whether a two-way call or a multi-party conference call.

What Is Trunk?

SIP transfers voice or video signals to a PBX, which in turn transfers them to the other end. The routes through which the internet protocol transfers the voice or video signals are called trunks—they are the roads that signals take to get from point A to point B. Trunks also have channels that carry incoming and outgoing calls.

What Is PBX?

A Private Branch Exchange System (PBX) allows multiple phones to communicate interchangeably—it connects with local telephone lines to make internal communication possible. This means that every phone line in an organization can make internal calls thanks to a PBX. The PBX for SIP trunking carries out the same functions; the only difference is that it uses cloud-based PBX instead of physical hardware.

Best SIP Providers

Using the right SIP service empowers business communication, and these leading companies are the top SIP trunk providers worldwide. Offering a variety of products and pricing models, these SIP trunk providing companies satisfy various business needs and purposes.


Twilio is a cloud-based communication platform that enables adding messages, voice, and video to your web or mobile applications. The platform offers products in three main categories: voice, video, and messaging.

In addition, Twilio provides solutions to businesses of all sizes including small, medium-sized, and large enterprises.

Main products:

  • Elastic SIP trunking
  • Internet of things
  • Messaging APIs
  • Programmable voice
  • Email API


Bandwidth is an innovative company and enterprise-trusted platform that has collaborative projects with Microsoft, Genesis, Zoom, and more.

Offering a wide range of products in communication API, Bandwidth easily implements voice, calling, text messaging, and emergency service access into software or applications—it directly connects public safety access to your app or software.

Main products:

  • Elastic SIP trunking
  • Enterprise 911
  • Voice API
  • Messaging APIs
  • Video API


Voxbone is a global CaaS company that enables call centers and conferencing platforms to transmit and receive phone calls, text messages, and faxes. The company provides Voxbone Voice, Voxbone Mobile, and Voxbone Insights to integrate compliant mobile voice into your CRM to reach customers.

Main products:

  • SIP trunks
  • Voice API
  • Messaging API
  • Emergency calling
  • Phone numbers


Plivo is a cloud communication platform that offers far-reaching products and channels to enable multimedia file transfers. Businesses can engage with their consumers with Plivo by creating compelling visual experiences that include images, music, and video.

Main products:

  • Zentrunk (classic SIP trunking)
  • Voice API
  • Number lookup

SIP Trunk Pricing: How To Choose the Best Option?

Trunk services come in a variety of pricing options—the cost differs from country to country and is based on case-by-case. Several factors determine the cost.

  • Establishing your business needs to choose the features you want to include
  • Deciding which package is best suited for your needs and your budget (monthly or annual subscriptions)
  • Recognizing the equipment required for SIP trunks. If you already have hosted PBX, the cost will be cheaper
  • Understanding the different pricing options based on your location
  • Learning about SIP providers like Plivio that redeem free credits and offer pay-as-you-go options or volume discount
  • Reaching out to your company of choice’s sales department to walk you through the purchase process

Hosted PBX Vs. SIP Trunking

Hosted PBX and SIP trunking models of business phone systems and virtual communication. On one hand, SIP trunking is a gateway that directly connects an existing on-site PBX system to the virtual network. On the other hand, hosted PBX is an offsite business VoIP phone system managed by the third-party Internet Telephony Service Provider (ITSP).

Let’s say your company has 100 employees and they need to be able to call each other. Ideally, they shouldn’t have 100 different phone numbers. Rather, they’d have extension numbers and the ability to forward calls. The PBX is the hardware that links between the company’s internal extension numbers, as well as customers and extension numbers.

As for external communication, the call first reaches the PBX, which sends it to the Public Switched Telephone Network (PSTN), and finally reaches the client.

Hosted PBX is essentially the same as PBX, but the equipment is hosted off-site by another company. PBX is expensive hardware, whereas hosted PBX is a relatively affordable option.

Hosted PBX provides:

  • Regular maintenance
  • Call features (call recordings, voicemail, transferring calls, and so on)
  • Software upgrades and service troubleshooting

If you already have hosted PBX, you can move your business communication to the cloud through a VoIP SIP provider.

SIP trunk provides:

  • Advanced phone system features like long-distance calling
  • Better call quality with relatively affordable cost

Which Type of Business Phone System Should You Install? - H2

Deciding on the right phone system greatly depends on your business needs, infrastructure, and working methods.

Larger enterprises usually opt for SIP trunking. If your company deals with international calls, involves more virtual work, high volume of calls, and has expansion potential, SIP trunking is the right choice for you.

Hosted PBX is ideal for small and medium-sized businesses that don't have a significant volume of phone calls.

SIP Trunk Vs. Elastic SIP Trunk

Besides hosted PBX and SIP trunking, businesses must also choose between elastic SIP trunk and SIP trunk services.

Elastic SIP trunking is a more advanced option of cloud communication that enables users to have more control over their needs. For instance, if your company faces an expected peak at a specific time of the year, you can pay-as-you-go instead of a monthly subscription.

This way, elastic SIP trunking:

  • Reduce costs
  • Provides calling flexibility
  • Meets fluctuating business needs