If you’ve been shopping for a business phone system and keep running into the terms “SIP line” and “SIP trunk” — you’re not alone. These two terms are often used interchangeably, frequently confused, and almost never explained in a way that helps a business owner make a practical decision. This guide fixes that.

By the end, you’ll understand what each term actually means, how they differ, which one your business needs, and the one thing neither of them does on their own — and what Phonewire does about it.

Jump to:

• What Is a SIP Line?
• What Is a SIP Trunk?
• SIP Line vs. SIP Trunk: Side-by-Side
• Which Should Your Business Use?
• The Missing Piece: What Happens When Internet Fails
• How Phonewire Handles SIP Trunking

What Is a SIP Line?

A SIP line is a virtual connection that enables a single VoIP phone or device to make and receive calls over the internet using the Session Initiation Protocol (SIP). Think of it as the digital equivalent of a single copper phone line — it connects one device to the outside phone network, handles one call at a time, and operates over your existing internet connection rather than physical wiring.

SIP is the technical standard governing how voice calls are initiated, maintained, and terminated over internet connections. When you place a call on a SIP-enabled phone, your device communicates with an Internet Telephony Service Provider (ITSP) — a company that connects your internet-based call to the traditional telephone network — and the ITSP routes the call to its destination.

How Does a SIP Line Work?

Each SIP line supports one call at a time. If your business has three SIP lines, you can handle three simultaneous calls — but a fourth caller gets a busy signal. The SIP line is the capacity unit: the more SIP lines you have, the more concurrent calls your system can handle.

In practice, individual SIP lines are most commonly discussed in the context of single-device setups — a single VoIP desk phone, a softphone app on a laptop, or a mobile VoIP app. For a business with a full phone system and multiple employees, SIP lines are typically bundled into a SIP trunk rather than purchased individually.

Who Uses SIP Lines?

SIP lines are well-suited for solo entrepreneurs, home offices, or very small businesses that need one or two phone lines without the overhead of a full PBX phone system. A freelance consultant, a single-operator service business, or a startup founder who needs a professional business number on a budget might start with individual SIP lines before growing into a full system.

What Is a SIP Trunk?

A SIP trunk is a virtual phone trunk that connects an entire PBX phone system to an Internet Telephony Service Provider (ITSP), enabling multiple simultaneous calls across the whole organization. Where a SIP line connects a single device, a SIP trunk connects the entire phone system — all extensions, all ring groups, the auto-attendant, voicemail, the works — to the outside phone network through a single logical connection configured for multiple concurrent calls.

The term “trunk” comes from traditional telephony, where a trunk line was a high-capacity connection between switching centers carrying many simultaneous calls. A SIP trunk is the modern, internet-based equivalent: a multi-channel connection between your PBX and your carrier that replaces the old T1/PRI circuits or bundles of copper POTS lines.

How Does a SIP Trunk Work?

A SIP trunk acts as the bridge between your PBX (whether on-premises hardware like Phonewire’s Hybrid system or a cloud-hosted platform) and the internet phone network. The trunk is configured for a specific number of channels — each channel can carry one concurrent call. A business configured for 8 channels can handle 8 simultaneous calls: inbound, outbound, or a mix.

All the advanced features of your PBX — auto-attendant, call routing, ring groups, voicemail, call recording — work through the SIP trunk. The trunk is simply the pipe; the PBX is the intelligence that decides where each call goes.

SIP Trunks and the POTS Retirement

If your business is currently paying for traditional copper phone lines (POTS lines) or a PRI/T1 circuit, SIP trunks are the direct replacement — and the transition is accelerating. AT&T is actively retiring copper infrastructure through 2029, with rate increases in some areas exceeding thousands of dollars per line per month as the network is wound down. SIP trunks deliver identical call capacity at a fraction of the cost, with no physical infrastructure to maintain. Phonewire handles POTS-to-SIP migrations as part of every phone system installation. See the full copper sunset guide →

Who Uses SIP Trunks?

SIP trunks are the right solution for any business with more than one or two employees who make or receive phone calls — which is virtually every business with a phone system. A 10-person office, a medical practice, a law firm, a manufacturing company, a nonprofit organization — all of these run their phone system’s external calling through SIP trunks, whether they know it by that name or not. If you have a modern business phone system, you already have SIP trunks.

SIP Line vs. SIP Trunk: Side-by-Side

In practice, the difference between SIP lines and SIP trunks matters most when you’re buying: individual SIP lines are for single-device setups, and SIP trunks are for any business with a full phone system. If Phonewire is installing your phone system, SIP trunk configuration is included — you don’t need to source it separately.

Which Should Your Business Use?

For most businesses evaluating a phone system upgrade, this is the practical answer: you need a SIP trunk. If you have more than two employees who make or receive calls, a PBX system with SIP trunking is the right architecture. Individual SIP lines are for single-device setups — they don’t scale to a team and don’t support the auto-attendant, ring groups, and call routing that a real business phone system requires.

Here’s how to think through your specific situation:

The Missing Piece: What Happens When Your Internet Fails

SIP lines and SIP trunks both share the same dependency: your internet connection. When that connection goes down — a broadband outage, a router failure, a power event that takes the ISP equipment offline — SIP-based calls stop working. Inbound calls can’t reach your system. Outbound calls can’t be placed. Your business number goes silent.

This is the single most common concern Phonewire hears from businesses evaluating VoIP: “We’ve had internet outages before. We can’t have our phones go down.” It’s a legitimate concern, and most VoIP providers address it with a workaround — call forwarding to a cell phone — which means your professional routing, auto-attendant, and ring groups all disappear the moment your broadband fails.

This is why many business owners instinctively search for a “traditional PBX” or “on-premises phone system” when evaluating options — they’re not necessarily attached to on-premises hardware for its own sake. They want a system that keeps working regardless of internet reliability. The Phonewire Hybrid delivers that, with modern SIP trunking and VoIP features, professionally installed on-site.

How Phonewire Handles SIP Trunking

When Phonewire installs a business phone system — either the on-premises Hybrid system or a cloud-hosted system — SIP trunk configuration is included as part of the installation. You don’t source SIP trunks separately, negotiate with a carrier, or configure technical parameters. Phonewire handles the entire SIP trunk setup: provisioning, number porting, channel sizing, and integration with the PBX.

For businesses migrating from copper POTS lines or PRI circuits, Phonewire coordinates the number porting process during installation. Your existing numbers transfer to SIP trunking in the background over 2–5 business days. The old lines stay active until cutover day. The new SIP trunk goes live when the new system does — no gap in service, no period where calls go missing.

For the Phonewire Hybrid on-premises system, SIP trunk capacity is typically sized at approximately $200/month for a business with 20 users and normal call patterns — significantly less than the equivalent capacity on copper POTS lines or a PRI circuit. See the full SIP trunking cost comparison →