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What is SIP Trunking and How Does It Work?

This article was updated on December 28, 2023

SIP (session initiation protocol) trunking allows your business to transmit audio and other types of media using an internet connection. Read on for a detailed breakdown of SIP trunking and what it can do for your business.

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 What is SIP Trunking?

SIP trunking makes it possible to transmit packets of media, such as voice, video, and chat, using an internet connection. Most often, SIP trunks are used to place calls over the internet; the SIP trunk functions as the gateway between the internet connection and the PSTN (public switched telephone network). As a result, SIP trunking can play nicely with both newer IP networks and legacy phone systems. It also has the capacity to allow your business to run a legacy PBX (Private Branch Exchange) phone system over the internet without needing to completely replace all existing hardware or connection points.


How Does SIP Trunking Work?

SIP trunking functions by creating packets that can transmit information from a PSTN to the internet or vice versa, thereby making online calls possible. And while SIP is perhaps most commonly used to transmit voice, SIP trunks are also capable of transmitting packets of information in other formats, including video and messaging. While traditional SIP trunking can have limitations in terms of scalability, newer elastic SIP technology allows your business to scale your usage on demand. 

Programmable SIP offerings, like the one offered at Vonage, make it possible to connect your PBX phone system or contact center to mobile, landline, SIP, and WebRTC endpoints with minimal work, including browsers and mobile applications. It can also bring Voice API functionality, such as multichannel recording, IVR, Text to Speech, WebSocket connectivity for AI integrations, and the power of contextual conversations to your platform.


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What are the Benefits of SIP Trunking?

SIP trunking can be used on-demand with no need to pre-provision, and it can provide voice connectivity with your existing PBX and contact center.

Elastic SIP trunking also offers your business the ability to scale your phone line usage up or down based on demand. Likewise, SIP trunking that scales with elasticity means that your business only has to pay for what you use. 

In essence, exceptional call quality and performance, PBX compatibility, and scalability are the key benefits of SIP trunking.


How Much Does a SIP Trunk Cost?

While pricing can vary for SIP trunking technology, elastic SIP can help ensure your business only pays for what you need. ​​

Some SIP Trunking providers, like Vonage, don’t charge for the SIP action itself but only for the PSTN and carrier connections (which vary by call type and country).  While Programmable SIP, which extends the SIP trunk to Voice API applications, does incur an additional charge, it also allows you to expand your voice channel functionality and create programmable experiences that can boost customer engagement.

SIP Trunking vs. VoIP

SIP trunking can be a foundational technology to power VoIP (voice over internet protocol). VoIP itself refers to any technology that facilitates voice calls using an internet connection. SIP trunking is one way that your business can power VoIP. And while SIP trunks can transmit packets of multimedia information, VoIP is only used to transmit voice calls.

 How Can You Choose the Best SIP Trunk Provider?

Here are some of the key considerations that should factor into your business’s decision when choosing a SIP trunking provider: 

  • Failover/Redundancy—Does the SIP trunk provider offer redundant phone lines so that if one fails, your call is not disconnected? Consider what options are available in the case of failover.
  • Scalability—An elastic SIP trunking solution adjusts the number of trunks used based on demand. This avoids the need to scale manually and can also provide cost savings as your business will only pay for the phone connections you need at any given time. 
  • Self-Service—Does the SIP trunk provider offer a dashboard or self-service portal so you can have insight into your set-up and quickly find some answers for troubleshooting? 
  • Call Quality—You’ll want to consider if the solution offers audio quality that ensures that your calls with either fellow employees or customers are clear.
  • PBX Compatibility—If you have a legacy phone system, you’ll want to ensure that the SIP provider is compatible with it. 
  • Programmable SIP Connectivity—If you’re looking to level up your SIP trunking solution and integrate customized voice experiences, you’ll want to find out if programmable SIP is an option. Programmable SIP technology offers a gateway between your SIP trunking-enabled phone system and a plethora of voice-enabled experiences powered by a Voice API.
  • Geographical Reach—Consider if the SIP trunking provider allows you to set up communications across geographic regions. For example, Vonage’s SIP trunking offers international numbers available in 72 countries with competitive pricing.


Exploring the Value of SIP Trunking for Enterprises

SIP trunking can bring a great deal of value in an enterprise where there's always more need for low-cost, high-impact digital communications solutions. Here are some additional benefits:

  • High or unlimited call volumes, including international calling
  • Centralized control, administration, and troubleshooting 
  • Reduced bandwidth usage, which can result in cost savings and better network performance
  • Integration with modern and legacy systems, allowing businesses to implement SIP with minimal initial disruption

Get Started with SIP Trunking

Ready to upgrade your legacy phone system? Explore Vonage’s SIP trunking solutions now.

Are you already using SIP trunking and want to level up your customer conversations using programmable SIP? Learn more about Vonage’s Programmable SIP solutions.

headshot photo of Steven Giuffre, Senior Product Marketing Manager for Vonage Voice API
By Steven Giuffre Senior Manager, Vonage APIs

Steven Giuffre is a Senior Manager for Vonage APIs. He leverages his Conversational AI and Voice expertise to promote personalized communications across channels for brands to deliver richer conversations and context to their customers. Steven lives in Los Angeles, where he enjoys beach life, films of most genres, Rams football, and his dog, Roxy.

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