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SIP Trunking – In depth Guide and Tips


Sip Trunking guide navigation:

  1. What is SIP Trunking
  2. How does SIP Trunking work
  3. Benefits of SIP Trunking
  4. Main Disadvantages
  5. SIP Trunk Migration Tips
  6. Is SIP the Right Choice


What is SIP Trunking?

SIP Trunking uses VoIP to connect a PBX and the Public Switched Telephone Network (PSTN). It is an application layer protocol that sets up audio or video sessions between two devices. As such it replaces the traditional phone trunking. SIP requires on-site PBX and a connection to your Internet telephony provider.

Trunking to a Hosted PBX is typically done using SIP. Another solution would be SIP PRI (Primary Rate Interface). When compared, SIP is the more scalable and profitable service. While PRI offers better quality of voice transfer but has some limitations to its usability.

With SIP trunks, companies can connect multiple channels to a cloud PBX. This allows companies to make simultaneous calls over the existent Internet connection. Without restrictions to the number of incoming or outgoing concurrent calls.

Why SIP Trunking:

  • Costs less
  • Scales easier
  • Is more reliable
“SIP is a well-established technology with minimal upfront investment costs”


How does SIP Trunking work?

SIP manages signalling and access to a Voice IP network. Trunks are a mediator between your phone systems and your Internet Telephony Provider. In other words it provides a connection between your telephony system and your provider of choice.

With SIP Trunking need of a physical connection to a phone company is obsolete. You no longer need that old wiring, or the circuit boxes used for connection to the PSTN.

“Just think of SIP trunking as a virtual version of an analog phone line.”

Benefits of SIP Trunking

“The cost savings and benefits of SIP trunking are hard to neglect.”

Main sell points of Sip:

  • Cost Savings
  • Disaster Recovery & Security
  • Cheap Long distance calls
  • Extremely Scalable
  • No infrastructure
  • Mobility
  • Remote access to lines
  • Reliability
  • Less constraints
  • Cost Savings


SIP trunking saves a significant sum of money reaching from 20 up to 60% of company telecommunications costs. Additionally it saves time and if implemented correctly reduces stress.

SIP trunks are virtual and provided over Internet making it easy to add or remove capacity. It’s easy to scale, add or remove team members when required, often at minimal costs and without hardware requirement.

Hardware and maintenance

Hardware and maintenance are almost virtual as well. No wiring is required, no on premise hardware is needed and maintenance is not mandatory. Even tech support is often remote.

Recovery and Security

In case of emergencies, Sip trunking is actually quite helpful. It is easy to setup a rerouting, and backing up with a spare internet provider is not that expensive. The service is Cloud based and does not have many of the limitations of on site PBX systems, especially when it comes to power losses. Almost every provider has a failover countermeasure and recovery strategy.

As every technology Sip trunking has its security exploits and vulnerabilities. Data safety is a big concern for most companies and should not be neglected. You can find a list of possible glitches here. Any Sip trunk provider should walk you through the possible breaches as part of the provided service.


SIP can be used with VoIP apps that come a very high degree of mobility. It allows company employees’ to connect with mobile devices or from remote computers. Some VOIP/SIP providers have even developed VoIP applications that require no data connection. Those apps can be used everywhere a mobile connection is available. With them mobile workers can remain connected to the communication system, and be reachable at all times.

Remote access to lines.

SIP supports remote access which is particularly handy for companies with offices based in multiple locations or countries. For outgoing calls it allows the use of all available numbers, no matter where you are located at the moment. Thus redirecting the incoming calls to your office of choice. SIP is extremely scalable, allowing startups and fast growing companies to open new sites or use full-time remote workers regardless of geographical location.

“You can make calls and still assure professional pick up in a different office or even in a different country.”

No Physical Infrastructure

“SIP trunking does not require additional hardware investment.”

Moving and expanding the company infrastructure before SIP trunking ware challenging. Both costed a hefty sum and ware lengthy processes. With SIP adding a new line is as simple as putting on a new pair of headphones and scaling your service agreement. Off-site workers are easily integrated with applications and can be reached with a simple extension number. Moving is fast and virtually free as no on-site hardware is required.


Traditional telephone service is often reliable. But it can still interrupt at times, because of construction works, bad weather conditions, or simple human errors. And if the line is broken there is no viable workaround. SIP trunking is a bit more reliable solution. Internet backup line is almost a must and guarantees calls will get through even if the main data provider fails. Cloud servers often have backups and can be restored within minutes, and vendors often offer mobile failover. Mobile VOIP applications can take over the SIP trunking functions until the service is restored.

Simple Management

Extensions management and functions like call pickup, call routing can be managed through simple administrative portals with a few clicks.

Most providers offer full control of all the outgoing and incoming calls, as well as real time monitoring and statistics. It is easy to check who used which phone with which extension, who was called and now long the calls lasted.

In addition, SIP trunking also:

  • Works with most modern phones
  • Can use use your old phone numbers
  • Can help you rationalise your telephony
  • Is perfect for seasonal companies
  • Can have flat plans

Main Disadvantages of SIP


Bandwidth Costs

Migrating all your communications to SIP always comes with the need of better bandwidth. Depending on the number of simultaneous lines that are to be used the additional cost might vary from a small sum to a small fortune. Despite the ne Internet connection costs, the final balance is still in SIP Trunkings favor.

Simultaneous Calls

When a company starts using SIP trunking it often chooses the wrong plan. The number of simultaneous calls is mostly ill-considered and needs to be re-assessed. Companies often miss-judge how many lines they will need and need to change their SIP and data plans.

“To determine the number of simultaneous calls you have to take into account both incoming and outgoing calls. Calls that exceed the limit will receive a busy signal and won’t wait in line for pickup.”

Porting Numbers

When switching to SIP all your DID numbers can be kept but have to ported to the new system. This process is not complicated but can take a good amount of time.

Possible SIP Trunking problems if it is not implemented correctly:

  • Call recording – can become messy
  • Fax – highly dependant on providers and might not be supported
  • 911 – proper implementation is required
  • Long carrier lead times
  • Number porting can take months
  • DTMF might not work

Successful SIP Trunk Migration Tips


Migration to SIP-Trunk


SIP trunking migration might become a complicated process if not planned and executed correctly. Here are some tips that will help the implementation:


Plan everything. Begin with inventory of the existing infrastructure. Number of phone lines, DIDs, check the overall usage both outgoing and incoming. Note if you will need to use the system from other locations. Don’t forget to include the backup system.

Key Decisions

SIP Trunking plans and carriers are flexible but you should make some firm decisions.

    1. Choosing the right voice compression between clearer sound and reduced bandwidth?
    2. Choosing the backup bandwidth (to fully cover or to be just for emergency)?
    3. Choosing what to do with the old hardware
    4. Choosing the carrier
“Choosing the Carrier for your SIP Trunking should not be left as the last decision. You should carefully review the available carriers, judging to what grade the offers they make are in line with your demands”

Expected Carrier Cost

In the end it is not all about the money. But the cost should not be neglected. Not all carriers list the following costs for comparison yet if asked they should provide the numbers:

Non recurring:

  • Initial Implementation
  • DID porting
  • Initial Training


  • DID numbers
  • Toll-free numbers and features
  • Number of lines
  • Caller Identification
  • Cloud Services
  • Support


Preparing for SIP

When you have determined what you need from your provider, and who it will be you should start planning the implementation. Every SIP provider has its course of action and you should make sure it is lined up with your needs. Be sure to take a test flight before you start the full implementation:

  1. Create a test plan
  2. Test the quality of service
  3. Create and test new DIDs
  4. Port few of your existing DIDs for testing
  5. Assign few checkers form your team that will gradually and troughtly test all new changes and options
  6. Test the internet bandwidth and backup
  7. To test the provided support, check time to action and availability
  8. Check the toll-free and 911 numbers for traffic and availability
  9. Check the control panel, reports and call logs


Potential Problems

As with every new undertaking, it is common for problems to arise. Consider several potential pitfalls areas that should not be neglected:

  1. Call recording, and storage might not work correctly or be messy
  2. Faxing might not work or need to stay on traditional line
  3. Tall free numbers and 911 numbers might not be connected or misbehaving
  4. Carrier screening or spoofing of outbound Caller IDs
  5. Carrier lead times might be too long
  6. Number porting might take too long
  7. Control panel might not work correctly
  8. Call logs reports and statuses might misbehave or be hard to read
  9. DTMF might not work

Is SIP Trunking the Right Choice?

SIP Trunking isn’t an alternative to hosted or on-premise PBX. It’s an alternative to publicly-switched telephone network. Is well-established and can lead to significant improvement in reliability and cost savings. The offered flexibility is a big plus but as any other solution it has more than a few disadvantages.
Still SIP Trunking is the right choice for a lot of companies which have left high priced and unreliable phone lines in the past and moved their business communications to the Cloud