A SIP trunk is a virtual phone line that connects a business phone system to the public telephone network via the internet, using the Session Initiation Protocol (SIP). It replaces traditional physical phone lines (POTS copper lines) and PRI/T1 circuits. Calls travel over your existing internet connection rather than a dedicated copper wire. SIP trunks typically cost significantly less than POTS lines and offer more flexibility — you pay for concurrent call capacity rather than a fixed number of physical lines.

What is the difference between SIP trunks and traditional phone lines?

Traditional POTS phone lines are physical copper wire connections, each carrying one call at a time, with a monthly fee per line regardless of usage. PRI circuits carry 23 simultaneous calls on a dedicated T1 connection at a fixed monthly cost. SIP trunks carry calls over your existing internet connection, charging only for the concurrent call capacity you need. A typical small business pays 70-85% less for SIP trunk service than for an equivalent number of POTS lines.

Can I keep my existing phone numbers if I switch to SIP trunks?

Yes. Your existing business phone numbers port to the SIP trunk service. Number porting typically takes 2–5 business days. Your old lines remain active during the porting window, so there is no gap in service. After porting, all calls to your existing numbers route through the SIP trunk to your phone system exactly as before.

How much internet bandwidth do SIP trunks require?

Each concurrent VoIP call uses approximately 80–100 Kbps of bandwidth using standard G.711 codec. A business with a 50 Mbps internet connection can support dozens of simultaneous calls with ample capacity remaining for data traffic. Phonewire assesses internet bandwidth and quality during the site survey before any SIP trunk installation.

Do I need to replace my phone system to use SIP trunks?

Most modern business phone systems — including Phonewire's P560 Hybrid on-premises system — natively support SIP trunks. Older analog PBX systems may require a VoIP gateway to interface with SIP trunks. Phonewire assesses compatibility during installation planning and includes SIP trunk configuration as part of every business phone system installation.

What Is SIP Trunking? Business Phone Lines Explained

SIP trunks are what replaces your telephone lines. Not the phones on your desk — the lines connecting your phone system to the outside world. If you’re paying a phone company monthly for each business line, you’re paying for something SIP trunks do better, for less money, with more features. Here’s what SIP actually is and why virtually every modern business phone system uses it.

What SIP Trunks Are — Without the Jargon

A traditional business phone line is a physical copper wire running from your building to the telephone company’s central office. Each line carries one call at a time. You pay for each line whether it’s in use or not. Adding a line means ordering a new circuit, waiting for installation, and paying more per month indefinitely.

A SIP trunk is a virtual phone line that runs over your existing internet connection. SIP stands for Session Initiation Protocol — it’s the technical standard that governs how voice calls are transmitted over internet connections. A SIP trunk connects your phone system (on-premises or cloud-hosted) to the telephone network via the internet rather than via dedicated copper wires.

The result looks the same to your staff and your callers: your phones ring, you make calls, the number is the same. The difference is entirely in the infrastructure — and the cost.

SIP Trunks vs. PRI and POTS Lines

POTS lines (Plain Old Telephone Service — individual copper phone lines) cost $80–$150+ per line per month in 2026, are being retired by AT&T and other carriers, and carry one call per line. A business with 5 POTS lines can handle 5 simultaneous calls and pays for all 5 lines regardless of how many are in use at any given moment.

PRI circuits (Primary Rate Interface — T1 lines dedicated to voice) carry 23 simultaneous calls on a single circuit. They were the standard solution for businesses that needed more than a few lines. A PRI costs approximately $400–$600/month in most markets, requires a dedicated connection, and still charges for all 23 channels whether you’re using them or not.

SIP trunks carry calls over your existing broadband internet connection. You pay only for the channels you use concurrently — or a flat monthly fee for a set number of concurrent calls. A typical small business with 10 employees might use 3–4 simultaneous external calls at peak and pays for that capacity rather than for a fixed number of physical lines. Phonewire’s SIP trunk service runs approximately $200/month for a business with 20 users and typical call volume — less than a single PRI circuit.

The math: A 15-person office paying $100/month for 8 POTS lines spends $800/month on phone line service. The same calling capacity on SIP trunks runs approximately $80–$120/month through Phonewire — the same concurrent call capacity, same features, same numbers, for 85% less per month on the line service alone.

How SIP Trunks Connect to a Phone System

Your phone system — whether it’s a physical PBX box in a closet or a cloud-hosted system — handles everything that happens inside your organization: extensions, voicemail, call routing, auto-attendant, ring groups. The SIP trunk is what connects that internal system to the external phone network so calls can go in and out.

On Phonewire’s systems, SIP trunk configuration is handled during installation. The trunk is set up as part of the system, tested, and documented before the installation is complete. You don’t need to manage it — if something needs to change (adding capacity for a busy season, adding a second number), you call Phonewire.

SIP trunks also support multiple phone numbers on a single trunk. If your business has a main number, a fax number, a direct line for a specific department, and a toll-free number, all of those can point to the same SIP trunk configuration rather than requiring separate physical lines for each.

What Changes and What Doesn’t

What stays the same: Your phone numbers. You keep every number you have — they port to the SIP trunk during installation. Your staff’s extensions, their voicemail, their call routing — unchanged. Callers reach you the same way they always have.

What changes: The monthly line cost drops significantly. You no longer have a physical copper or fiber circuit for voice — calls travel over your internet connection alongside your data. You gain capacity flexibility — adding concurrent call capacity is a configuration change, not a physical installation.

What you need: A reliable internet connection with adequate bandwidth for the number of simultaneous calls you expect. As a rough guideline, each concurrent VoIP call uses approximately 80–100 Kbps of bandwidth — a 50 Mbps business connection can handle dozens of simultaneous calls with capacity to spare for data traffic. Phonewire assesses your internet capacity during the site survey before any installation.

SIP Trunks and the POTS Retirement

AT&T and other major carriers are actively retiring copper POTS lines through 2029. Businesses still paying for POTS lines are facing two realities simultaneously: their existing lines are being grandfathered off with rate increases that in some cases exceed $2,700/line/month, and when their area’s wire center is decommissioned, the line is simply gone with 90 days’ notice.

SIP trunks are the direct replacement for POTS lines on business phone systems. Phonewire replaces your POTS lines with SIP trunks as part of any phone system installation — or as a standalone project if you want to retire the POTS lines from an existing system. Your numbers port, your system connects to the SIP trunk, and the monthly copper line cost goes away. See the full POTS retirement guide.

To discuss SIP trunking for your specific setup — how many lines you have, what your current costs are, and what a transition would look like — call Phonewire at (800) 857-1517 or schedule a free consultation.