Public switched telephone networks (PSTN) are dying, thanks to long-distance fees and expensive hardware. There’s also the audio quality issues that are often experienced on landlines, which makes for unclear audio conferences and calls.

SIP trunking is quickly becoming the go-to technology for businesses looking to make clear calls and conferences. According to a recent study, 90% of SIP users report that they are satisfied with call quality. Still, SIP trunking has its issues, which is why many businesses are opting to switch to elastic trunking instead. 


What is Elastic SIP Trunking?

Elastic SIP trunking is a communications option that’s a bit more advanced and versatile compared to standard SIP trunking. Elastic SIP trunks use a cloud-based virtual connection to add flexibility to the SIP trunking experience. For example, with standard SIP trunks, you’re typically paying for the maximum number of concurrent calls. Even if your calls fall well below that amount, you’re still paying for the top end rate.

With elastic trunks, there’s a pay-as-you-go style of billing, so you pay for what you need without any extra charges. This is ideal for businesses experiencing higher call volumes during specific times of the year. It’s also useful for call centers that need to increase or scale back call capacity during different campaigns. This is one of the key reasons why elastic SIP trunks have the “elastic” moniker – they are extremely flexible for businesses that need it.


Since elastic SIP trunks are cloud-based, it’s easier for your system to integrate with other cloud services. This is useful for larger organizations with a unified system that incorporates the same cloud services into their architecture around the globe. For example, your office in London, UK, will have the same SIP calling service as the office in NY, NY. With elastic SIP, you can manage both cloud PBX users and SIP trunking users through the same portal.

The portal itself is more complex than what’s available with standard SIP trunks. It serves as a mission control that you can use across your entire organization. Details like usage statistics, currently active calls, and traffic levels are all available through most elastic trunking software. Some software, like that from Telnyx, has call flow visualization, provisioning, and codec selection so that your company has direct control over call audio and video quality.


How it Works:


Elastic SIP trunks work like standard SIP trunks because they connect IP channels to the traditional PSTN or other VoIP-based communications channels. This means that your business easily calls another company that’s using an elastic SIP or reach customers on their landlines. SIP trunks either terminate or originate calls.  Call termination is the connection of your existing SIP infrastructure so that you can make calls out to the PSTN. Origination, on the other hand, is receiving calls from the PSTN on your SIP trunk-connected phone lines.

Elastic SIP trunks have some unique features that you may not find on traditional trunking solutions. For example, direct inward dialing (DID) numbers, which are also called virtual phone numbers, are available through elastic systems. Vanity numbers, which would look something like 1-800-YOURBIZ, are also easily provisioned by the provider to make your business associated with a memorable number.

Virtual numbers can be toll-free (1-800, 1-877, 1-866, etc) or local so that customers in remote areas can reach your business in their area code. Your business may be located across the country, but users in distant markets will think that they are reaching your business locally. This is even available in international territories. Elastic providers like Twilio grant numbers in over 50 countries.


What are the Benefits?

One of the best benefits of elastic SIP trunking is call concurrency. There are no limits when you’re working with an elastic system, so businesses like call centers quickly provision new lines when they are expanding. When thinking of the advantages, think of the benefits of upgrading from a primary rate interface (PRI) to a SIP-based system. 

With PRI, you’re limited to branches of 23 lines when you’re looking to upgrade. With standard SIP, you aren’t limited, but you’ll be purchasing connectivity that you won’t be using when calling is light. Elastic trunking serves as a diametrical shift in this regard; instead of running the chance of not using purchased calling capabilities when calling is reduced, you pay only for what you use. 

The cloud-based nature of elastic SIP trunks also makes it more convenient to manage. They’ll scale with your company thanks to SIP trunk IT teams being able to change calling capacity in real-time. This helps you avoid rejecting calls due to limited calling capacity. The percentage of calls blocked is a vital call center metric to track because reducing this number increases customer satisfaction, so having an unlimited capacity for calls that adapts to call volume is beneficial.

All SIP calling uses SIP uniform resource identifiers (URIs) to open connections for both termination and origination. These look almost like email addresses since they use both the @ symbol as well as .com in many situations. These also work like web addresses and are placed in locations where your customers access them like web pages and emails. 

Most elastic providers provide calling from SIP URI to SIP URI at no additional cost, which means significant cost savings when communicating with other businesses that use SIP trunking. Pure SIP URI calling is completely separate from SIP calling to PSTN lines.


Elastic SIP Trunking vs Basic SIP Trunking

SIP Trunking Elastic Trunking
Concurrent Call Capacity Dependent on plan Unlimited and based on what you use
Connection Requirements On-premise hardware Internet connection
Average Dialing Pricing $25 to $50 per trunk, per month $0.005 to $0.009 per minute
SMS Reception Through API Included with service ($0 to $0.025 per message) 
Outbound SMS and MMS  N/A Available in some global markets
Average Setup Costs $0 to $175 per trunk Typically $0
Provisioning Done through the provider Done through the interface
Billing Schema Based on the number of lines on the trunk Based on the amount of calling done during the billing period. 
Pricing Style Billed per channel as a flat rate per month Billed based on usage – typically per minute
Real-time Call Monitoring N/A Usage statistics, traffic levels, and active calls
Geographic Flexibility Limited Global reach across offices
Maintenance and Management On-staff IT Performed by provider staff
Critical Functions Calling Calling

Emergency calling


Server georedundancy

Call routing

Failover and load balancing

Routing Randomized Location-based to reduce latency
Bring Your Own Carrier (BYOC) N/A Available from several providers
Internet of Things (IoT) compatibility N/A Available from some providers over LTE networks
Multi-tenanted SIP Trunks N/A Available through most providers
Call Recording N/A Available through most providers


Provider Comparison: The Top Elastic SIP Providers




Twilio’s elastic trunking gives your business flexible connectivity and functionality in minutes. Their cloud structure allows you to connect on a global scale using unlimited concurrent calling. Twilio elastic SIP trunking supports both small offices and enterprise offices with multiple locations.  Some of the SIP trunking features that are included with Twilio’s offerings are:

  • Private connections to communicate
  • Emergency calling that will correctly route 911 calls
  • Security that has encrypted signaling with transport layer security and secure real-time transport protocol
  • Toll fraud protection
  • Monitoring for issues with packet loss, jitter, and other metrics
  • Troubleshooting of individual calls
  • Notifications to allow you to respond to issues more quickly
  • Failover across multiple PBXs
  • Disaster recovery via the cloud
  • Multichannel experience that includes SMS and MMS
  • Built-in call recording without additional hardware


With Twilio, you are only charged for what you use based on the origination and the termination of your call. Twilio pricing looks like this:

Type of Calling Termination Price Origination Price
Local Numbers $0.0070/min $0.0045/min
Toll-Free Numbers $0.0070/min $0.0180/min
Zone 2 – Hawaii $0.0140/min $0.0045/min
Zone 3 – Alaska $0.0500/min $0.0045/min
Zone 4 – US $0.1000/min $0.0045/min
Zone 5 – US $0.2500/min $0.0045/min


Additional charges for Twilio include:

Service Charge
Private Connections $0.0010/min
Emergency Calling $1.00/line
Call Recording $0.0025/min
Recording Storage $0.0005/min




Another cloud-based elastic trunking provider that’s ideal for scalability in business is Bandwidth. They provide companies with high-quality simultaneous calling across the United States that can easily be upgraded remotely if the need arises. Some of the features offered to enterprises through Bandwidth include:

  • International long-distance and toll-free calling
  • Emergency 911 calling based on the location
  • Caller ID information when you receive a call
  • 24/7 network operations center 
  • Disaster recovery
  • IP failover control options
  • Porting support
  • Number management 

When your business goes with Bandwidth, you only pay for the calls and the features that you use. A number will cost $0.35 per line each month, but you will have an additional fee for the cost of calling and sending messages that looks like this:

Type of Contact Pricing
Inbound Calls $0.0055/min
Outbound Calls $0.01/ min
Incoming SMS No Charge
Outbound SMS $0.005/message




Telnyx is a provider that has global elastic trunking that requires minutes to setup. Telnyx has a high level of security that most businesses appreciate for their communication. The data sent through elastic trunking is encrypted, and it uses TLS, SRTP, and ZRTP to make sure that it remains secure during travel. Some features included with Telnyx are:

  • Fraud and spoofing protection
  • Emergency calling that’s routed to the nearest public safety answering port
  • Automatic call recording and cloud storage
  • Detailed diagnostics and call statistics
  • Alerts for suspicious activity
  • Custom routing for better call flow
  • Track of porting requests 

With Telnyx, your business will only pay for what you use, but the more you use, the lower the rates will be. With 1 to 100,000 calls a month, local outbound calls will be $0.0075 per minute, and toll-free calls are free to make. Incoming local calls cost $0.0075 per minute, and toll-free calls cost $0.015 per minute to receive. As the number of calls increase, the pricing structure looks like this:

Number of Calls Outbound Local Calls Inbound Local Calls Incoming Toll-Free Calls
100K – 1M $0.0075/min $0.0064/min $0.012/min
1M – 10M $0.0075/min $0.0051/min $0.01/min
10M – 50M $0.006/min $0.0031/min $0.009/min
50M – 100M $0.006/min $0.0024/min $0.008/min
100M+ $0.005/min $0.002/min $0.007/min

Each channel supports one concurrent call, so if you need to make multiple calls simultaneously, more channels need to be purchased. 1 to 10 channels are $12 a month, 10 to 50 channels are $11 a month, 50 to 250 channels are $9 a month, and more than 250 channels brings the price down to $8 a month. Calls that need to be recorded will cost an additional $0.002 each minute.

With this SIP trunking provider, you will also have to pay for the numbers that you use. The more numbers that you need, the less it will cost for each local and toll-free number.

Number of Lines Cost per Month
1 – 50 $1/line
50 – 250 $0.79/line
250 – 1K $0.50/line
1K – 5K $0.39/line
5K+ $0.20/line



Vonage is an elastic provider that combines SIP and cloud PBX for businesses that have multiple global sites. The service has critical features for business like call recording, fax, auto-attendant, international virtual numbers, and on-demand call analytics. This provider delivers elasticity when it comes to SIP-enabled devices – their service is compatible with the majority of desk phones. 

Vonage advertises its elastic SIP trunks for financial services, healthcare, travel and hospitality, logistics and transportation, and retail and e-commerce. Features like customer two-factor authentication, fraud detection, and video and voice APIs are easily used in all of these industries.

Here are some of the features that make Vonage unique:

  • Per-second billing
  • Built-in fraud detection
  • Automatic location-based routing
  • Chat across connected devices
  • Customer journey tracking
  • Integration with third-party vendors including AI bots
  • Call recording and dashboard analytics 
  • WebRTC-based contact center calling

Vonage is the only provider with per-second billing, so users that want a truly fine-tuned pay-as-you-go trunking solution may prefer this provider because its billing is very cost-effective. Vonage charges $0.98 per virtual number, per month, but call pricing looks like this:

Type of Call Pricing
Outbound PSTN $0.0139/min
Outbound SIP $0.0044/min
Outbound Virtual Number $0.0139/min
Incoming PSTN $0.0049/min
Incoming SIP $0.0044
Incoming Virtual Numbers #0.0049



As a provider, Flowroute has several elastic features such as instant number activation and a self-service platform for users that make it a good option for SIP. Flowroute has a global reach that covers North and South America, Europe, and Asia. They also have a “five nines” uptime guarantee, which equates to about five minutes of downtime a year. Their pay-as-you-go pricing plans include toll-free SMS and MMS, and there’s the option for both local and international virtual numbers.

Here are some of Flowroute’s key takeaways:

  • REST APIs on a management platform
  • Self-service web portal with analytics
  • Virtual numbers in more than 160 countries
  • A la carte pricing for extra features
  • Instant number activation
  • 99.999% uptime thanks to the provider’s HyperNetwork technology
  • Long code and toll-free SMS and MMS
  • Fax over IP networks

Flowroute has four pricing tiers that include the Self-Service, Silver, Gold, and Platinum plans. Self-Service is their only SIP trunking plan with by-the-minute pricing. Additional phone numbers also cost $0.50 per month. Flowroute’s pricing structure looks like:

Type of Contact Pricing
Incoming Local Calling $0.00500/min
Outbound Local Calling $0.00833/min
Toll-Free Incoming Calling $0.00975/min
Long-Code SMS $0.004/message
Toll-Free SMS $0.0075/message
Long-Code MMS $0.095/message
Toll-Free MMS $0.019/message


Gain Better Calling Flexibility with Elastic SIP Trunking 

SIP trunking isn’t going anywhere. It’s clearly replacing PSTN lines since telecoms are reporting that they are losing nearly a million customers per month to more agile technologies like SIP. SIP is also more versatile than most business communications phone systems. It provides the backbone for both VoIP and unified communications, so embracing a SIP trunking provider yields dividends.

Elastic trunking only improves the flexibility of SIP. It’s particularly useful for businesses with high call volumes, like call centers, and companies that want a more dynamic calling plan. Its ability to link multiple, diverse offices together under a cloud-based calling umbrella also delivers value to larger companies. 

To learn more about this technology as well as other enterprise telephony solutions solutions, take a look at our guides on the top providers and check out our tables to learn more about their features and pricing.