SIP Trunking: How to Upgrade Your Telephone Service Without Replacing Your PBX

Key Takeaways

• SIP trunking replaces traditional POTS telephone lines with internet-based calling while allowing your existing on-premise PBX hardware to remain in place
• Businesses that switch from POTS to SIP trunking typically reduce per-line monthly costs by 40-60% while gaining call quality improvements and modern features
• SIP trunks are virtual, meaning you can add or remove lines in minutes without a technician visit — no physical wiring required
• According to Technavio, the global SIP trunking services market is projected to grow by $11.8 billion between 2022 and 2027 as POTS retirement drives migration
• SIP trunking is the right middle path for businesses that have recently invested in on-premise PBX hardware and are not yet ready for a full cloud system replacement

If your business has an on-premise PBX and is facing rising landline costs, SIP trunking is often the most practical next step — not a full system replacement, not staying with analog service, but a targeted upgrade that changes how calls travel while preserving your existing hardware investment.

What Is SIP Trunking?

SIP stands for Session Initiation Protocol — the standard communication protocol used to establish and manage VoIP calls. A “trunk” in telephony terminology is a connection that carries multiple simultaneous calls. In the traditional world, trunks were physical phone lines connecting your PBX to the public telephone network. SIP trunks do the same thing over the internet.

When you add SIP trunking to your existing PBX, you are replacing the physical POTS lines that connect your system to the outside world with internet-based connections that use the SIP protocol. Your PBX continues managing internal extensions and call routing exactly as before. The difference is that outgoing and incoming calls now travel over your broadband connection rather than copper wire.

This is distinct from a hosted PBX telephone service, where the entire PBX is moved to the cloud. With SIP trunking, your hardware stays on-site — you are only changing the outbound calling pathway.

Who Should Use SIP Trunking vs. a Cloud Telephone Service?

The choice between SIP trunking and a fully hosted cloud telephone service comes down primarily to where you are in your hardware lifecycle.

SIP trunking makes sense when:

• You have on-premise PBX hardware that is 2-5 years old and in good working condition
• Your PBX vendor supports SIP connectivity (most modern systems do)
• You want to reduce monthly telephone service costs without the disruption of a full system replacement
• You have an IT team comfortable managing on-premise hardware

A cloud phone system or hosted PBX makes more sense when:

• Your on-premise hardware is aging and approaching end-of-life
• You want to eliminate server room equipment and maintenance entirely
• You have remote or multi-location teams that need a unified system
• You are starting fresh with no existing PBX investment to protect

For a direct comparison of both approaches alongside traditional POTS, the VoIP vs. traditional phone systems analysis breaks down costs, features, and use cases. With the rise of digital communication, it’s important to know how to keep your business number with VoIP to maintain a professional image. Many companies are making the switch to ensure flexibility and enhanced features that traditional systems cannot provide. By leveraging VoIP, businesses can also benefit from significant cost savings while improving customer accessibility. upgrading network capacity for voip is crucial for businesses looking to enhance their communication systems. By ensuring sufficient bandwidth, organizations can better handle increased call volumes and provide higher quality audio. This is particularly important in today’s fast-paced digital environment where seamless connectivity can significantly impact customer satisfaction. One major consideration in this analysis is the benefits of cloud phone systems, which often include scalability, lower upfront costs, and enhanced features such as call recording and analytics. Businesses can adapt quickly to changing needs with cloud solutions, ensuring they remain competitive in today’s fast-paced environment. Additionally, these systems typically offer greater reliability and support for remote work, making them an attractive choice for many organizations.

How SIP Trunking Works

Your on-premise PBX connects to a SIP trunk provider through your internet connection. The SIP trunk provider authenticates your system, assigns your phone numbers to the virtual trunks, and routes calls in both directions.

Incoming calls arrive through the internet, are recognized by your SIP trunk provider as belonging to your number, and are forwarded to your PBX over the internet connection. Your PBX then routes the call to the appropriate extension as it always has.

Outgoing calls from your PBX travel over the internet connection to the SIP trunk provider, which converts them to whatever format the recipient’s network uses and completes the call — whether that destination is another VoIP user, a mobile phone, or a traditional landline.

The technical overview of how VoIP phone service works explains the SIP protocol, codec selection, and call routing in more detail.

What SIP Trunking Costs

SIP trunking pricing varies by provider, but the cost structure is typically simpler than traditional telephone service:

Per-trunk pricing: You pay for the number of concurrent calls you need. A business where 5 people might be on calls simultaneously needs 5 SIP trunks. Adding a trunk is an account change, not a physical installation.

Per-minute pricing: Some providers charge per minute of outgoing calls rather than a flat monthly rate. This can be cost-effective for businesses with low outgoing call volume.

Bundled pricing: Flat monthly rate for a defined number of trunks with unlimited domestic calling included. This is the most common model for small and mid-size businesses.

The VoIP business phone system pricing guide includes SIP trunking pricing context alongside hosted PBX pricing so you can compare total costs across both options.

Network Requirements for SIP Trunking

SIP trunking puts your voice traffic on the same network as your other internet traffic, which means network configuration matters. Poorly configured networks produce call quality problems — jitter, echo, dropped calls — that are not caused by the SIP trunking service itself.

Bandwidth. Plan for 100 kbps of dedicated bandwidth per concurrent call. For a business with 10 SIP trunks, that is 1 Mbps allocated specifically to voice traffic — manageable on most business broadband connections.

QoS (Quality of Service). Your router must be configured to prioritize voice packets over other traffic types. Without QoS, a large file download or video stream can degrade call quality for everyone on a call at that moment.

SIP ALG. Many consumer-grade and some business routers have a feature called SIP Application Layer Gateway (SIP ALG) that is supposed to help with VoIP but frequently causes call quality problems. This feature should typically be disabled when deploying SIP trunking.

Firewall rules. Your firewall must allow the specific ports used by SIP signaling (typically port 5060 or 5061 for encrypted SIP) and the RTP ports used for audio transmission. The office network preparation checklist for VoIP covers all of these configurations in detail.

Number Porting with SIP Trunking

Your existing business phone numbers can be ported to your SIP trunking provider the same way they port to any telephone service provider. The numbers map to your virtual trunks, and calls to those numbers route to your on-premise PBX over the internet connection. No number changes, no customer communication required. The business phone number porting process ensures that your company can maintain its established identity while upgrading to a more efficient communication system. This seamless transition minimizes downtime and maintains customer trust, as they can continue to reach you on the same numbers they are familiar with. By simplifying the porting process, you can focus on what matters most—growing your business.

SIP Trunking and POTS Line Retirement

For businesses on POTS service, SIP trunking is one of the two main migration paths as copper networks are retired (the other being full hosted VoIP). If you have an on-premise PBX that was previously connected to POTS lines, SIP trunking is the path that preserves that hardware investment while completing the move away from analog service.

The guide to POTS line retirement and copper sunset migration explains the timeline and options in more detail for businesses currently on analog service.

Limitations of SIP Trunking

SIP trunking is not a complete solution for every business. A few limitations to be aware of:

Features are limited to what your PBX supports. The features available to your team are determined by your on-premise PBX firmware and licensing, not by the SIP trunk provider. If your PBX does not support a feature you want, SIP trunking will not add it.

You still own the hardware. When your PBX breaks, repairs and replacement are your responsibility. With a hosted PBX, hardware maintenance is the provider’s problem.

Remote and mobile users are harder to support. Without cloud infrastructure, extending your system to remote workers typically requires VPN connections and additional configuration. Cloud-hosted systems handle this natively through mobile apps.

Disaster recovery is your responsibility. With on-premise hardware, a local power outage or equipment failure affects your entire phone system unless you have specifically built redundancy into your setup.

Frequently Asked Questions About SIP Trunking

Is SIP trunking the same as VoIP?

SIP trunking is a specific method of delivering VoIP calls to an on-premise PBX. All SIP trunking uses VoIP technology, but VoIP is the broader category. Hosted PBX and cloud phone systems also use VoIP but deliver it differently. The complete business telephone services guide explains how all of these options relate to each other.

Will SIP trunking work with my existing PBX?

Most modern on-premise PBX systems from manufacturers like Avaya, Cisco, Mitel, and 3CX support SIP trunking. Older analog or proprietary systems may not. Your PBX documentation or vendor support can confirm compatibility. Vistanet’s team checks this during the initial needs analysis.

How many SIP trunks do I need?

The general rule is one trunk per concurrent call. If you expect no more than 10 simultaneous calls at peak times, 10 trunks is the right number. Many providers offer burst capacity so you are not hard-capped at your contracted number of trunks during unexpected spikes.

What happens if my internet goes down?

With SIP trunking, your calls depend on your internet connection. Configure your system to automatically forward calls to a cell phone or answering service when internet connectivity is lost. A disaster-ready communication plan documents this backup routing in advance.

How do I find out if SIP trunking is the right choice for my business?

A free needs analysis covers your current system, hardware status, call volume, and business requirements and tells you directly whether SIP trunking or a hosted cloud system is the better path. Contact Vistanet to schedule yours.

If you are mapping out your communications from the ground up, start with building a business telecommunications plan.