What is SIP Trunking?

SIP Trunking is a method by which business phone systems can operate using an internet connection instead of a traditional phone line.

SIP Trunking:

  • Costs less - Saves money on your business phone
  • Scales easier - Makes it easy to scale your operation pain-free
  • Is more reliable - Offers increased phone service reliability

SIP trunking is provided by SIP providers, which are similar to traditional phone companies, except SIP providers give access to phone lines over the internet (and SIP service is significantly less expensive - sometimes as much as 60-80% less).

Learn more about the benefits of SIP trunking for your business.


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What is SIP trunking?

Many companies already use VoIP within their phone system on the Local Area Network (LAN) to connect to IP phones.

SIP Trunking also uses VoIP to take advantage of shared lines, such as a company’s Internet connection, to allow more flexibility in communications.

Traditional phone systems (that aren’t already VoIP-capable) can be connected using common VoIP gateways to take advantage of SIP trunking and reap the significant cost benefits.

How does SIP Trunking work?

SIP trunking works with VoIP phone systems (Voice Over Internet Protocol) and is based on SIP (Session Initiation Protocol).

SIP, which is the basis of SIP Trunking, is the standard communications protocol for voice and video in a Unified Communications (UC) solution across a data network.

How does SIP trunking work?

SIP Trunking eliminates the physical connection to a phone company. There are no hardware, wiring, or circuit boxes to maintain for connection to the PSTN (phone service provider).

A SIP “trunk” is installed virtually over your business’s existing internet connection, therefore replacing the need for traditional analog phone lines.

Reducing multiple phone lines into a single point of entry drastically reduce charges for incoming lines and the IT cost associated with the maintenance of those lines. (Some organizations prefer to maintain standard lines for faxes and alarms.)

A phone number, or Direct Inward Dialing number (DID), is less expensive when purchased with a SIP Trunk.

Traditionally, when a DID is obtained from a phone company, charges are applied for the DID, IT and maintenance services, and the hardware connecting the shared physical lines or channels. A DID provided without these infrastructure costs is more affordable.

Benefits of SIP Trunking

Cost Savings

The cost savings and communications benefits of SIP trunking are substantial.

If your business is experiencing:

  • High monthly phone bills
  • Costly charges for incoming phone lines
  • Long distance charges
  • IT and maintenance fees
  • Constraints of limited communications technology

...SIP trunking can likely help in a big way.

SIP Trunking allows companies to only pay for the number of lines they need as opposed to getting locked into excess analog lines or partially-used T1s and PRIs. The savings are realized either by purchasing only the necessary number of channels or by paying only for minutes used. This allows companies to make more efficient use of communications costs and reduce or eradicate wasted resources.

In addition to cost savings, SIP trunking also provides:

  • Scalability
    • Because SIP trunking uses a virtual connection, adding lines or modifying service is fast and simple.
  • Reliability
    • SIP Trunking with VoIP increases reliability of services by providing a level of redundancy. When system failures and emergencies occur, SIP Trunking providers can reroute services to a redundant data line or forward the PBX to mobile phones to keep your business up and running. Start using in minutes
  • Works with most modern phones
  • Ability to transfer and use your old phone numbers
  • Affordable phone numbers for all your team members
  • Less maintenance - No longer have to maintain analog and IP infrastructure

Call to ActionSee how SIP trunking can save you money and improve your business with our free on-demand webinar. Watch Now!

SIP Trunking vs. PRI

If you’ve been shopping around for business phone systems, you’ve probably come across the term PRI (Primary Rate Interface). Often times you’ll hear people compare PRI to SIP Trunking, and it’s important to know the pros and cons of each before making a decision.

So what is PRI and how does it compare to SIP Trunking?

Basically, PRI and SIP Trunking are two different ways to “connect” your phone system to the outside world. On the most basic level, SIP Trunking uses your internet connection to make phone calls, while PRI uses a dedicated physical connection.


Uses Traditional Phone Lines

SIP Trunking

Uses existing internet connection


  • Quality of Service is guaranteed
  • Works with older phones that aren’t compatible with VoIP


  • Typically more expensive than SIP
  • Connections are physical and new lines have to be installed when your company grows
  • Require in-house IT staff to maintenance


  • Usually more cost-effective for businesses
  • Scalability - quickly and easily modify  service
  • Connections are virtual, so no physical lines


  • Quality of Service is not guaranteed, as internet connections can introduce lag and delay if not configured correctly or there is insufficient bandwidth.
  • Does not work natively with older analog or digital phones

SIP Trunking vs. VoIP

Simply put, SIP is a protocol that helps enable VoIP phone systems. So while the two terms can seem like different services, they actually work to accomplish the same goal - to run your business phone system over your internet connection and internal data network.

Check out our VoIP Basics page to learn more about how VoIP works and how it can benefit your business.

Call to ActionReady to learn more about how SIP trunking can save you money and make your business more efficient? Watch Now!

Things to know before shopping for SIP Trunking service

To prepare your company for SIP Trunking, you first need to consider a few questions.

How many people are on the phone at the same time during your busiest hours?

That will determine how many channels you will need. Remember, SIP trunks allow for quick and easy scaling, so you may add or remove channels as needed if you under or overestimate.

Will your call volume remain fairly consistent from week to week, or will it fluctuate?

With SIP Trunking, you can either purchase by the channel or by the minute. Pricing is explained more on our SIP trunking pricing page.

Will you need to scale or add new users relatively quickly?

This is an important factor, particularly when deciding between on-premises and hosted phone systems. Here's a quick guide to help explain the difference between on-premises vs. hosted.

Another thing to think about
is whether or not your network is ready for SIP Trunking.

Network considerations that must be examined include total available bandwidth, Quality of Service (QoS), and firewalls. (See below for more details.) Upgrading your Internet connection may be necessary to ensure sufficient bandwidth to carry a new phone system on top of typical Internet usage for your company.

Use the following simple equation to determine the necessary bandwidth to support your calls:

VoIP Bandwidth - Number of Concurrent Calls Image

(number of concurrent calls at your company’s peak)


VoIP Bandwidth - 100kb per Second Image

100 kilobits per second


VoIP Bandwidth - Bandwidth Image

bandwidth in Megabits per second (Mbps) needed for calls

Digium Solutions SIP Trunking Switchvox Switchboard and D80

QoS: Equally important to bandwidth is QoS (Quality of Service). QoS prioritizes your voice traffic and ensures that your phone calls are going to get the bandwidth needed, regardless of what else is happening on the network. The vast majority of business grade network routers will provide QoS for your network.

Firewall: A firewall is critical to maintain security both within a LAN and Wide Area Network (WAN). Though firewalls are a critical component to any business network, they must be configured correctly to work well with SIP trunking.

E911: For safety, it is essential to add Enhanced 911 (E911). E911 is a feature of the 911 emergency-calling systems that places VoIP emergency callers with the appropriate resources by associating a physical address with the calling party's telephone number.

If you’re unsure what any of this means, don’t worry.

Our free network testing tool can assess your current network quickly and easily, that way you’ll have a better idea whether or not cloud or SIP Trunking is right for your business.

Check your network now

SIP Trunking Pricing

SIP Trunking pricing varies depending on whether you want to pay per minute or per channel.

Per minute pricing, also known as "metered" pricing, works well for businesses that need more flexibility in their communications from week to week.

Per channel pricing, also known as "channelized", is great for businesses that prefer a set, predictable monthly phone bill.

To see how much SIP Trunking costs, check out our SIP Trunking Pricing page.

Next Step:

See how SIP trunking can save you money and improve your business with our free on-demand webinar "Everything You Need to Know about SIP Trunking"

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