What Is a SIP Trunk and How Does It Work?

SIP Trunking in Plain Language

A SIP trunk is a virtual phone line that connects your business phone system to the public telephone network (PSTN) over the internet. Instead of physical copper wires running to your office, a SIP trunk uses your existing internet connection to carry voice calls.

Think of it as the bridge between your internal phone system and the outside world — the piece that lets you dial regular phone numbers and receive incoming calls from landlines and mobile phones.

How SIP Trunking Works

The process involves three layers:

1. Your Phone System (the endpoint)

This is where calls originate and terminate. It can be:

• A SIP softphone app (like Softphone Plus) on your computer or mobile
• A hardware IP phone on your desk
• A PBX (Private Branch Exchange) managing multiple extensions

2. The SIP Trunk (the connection)

The SIP trunk provider connects your phone system to the PSTN. When you dial a number:

1. Your softphone sends a SIP INVITE message to the trunk provider
2. The provider routes the call to the destination via the PSTN
3. Audio flows as RTP (Real-time Transport Protocol) packets over your internet connection
4. When either party hangs up, a SIP BYE message terminates the session

3. The PSTN (the destination)

The public telephone network that connects to landlines, mobile phones, and other businesses worldwide.

SIP Trunk vs. PRI vs. Analog Lines

When Do You Need a SIP Trunk?

You need a SIP trunk if:

• You have an on-premise PBX (like Asterisk, FreePBX, or 3CX) and need to connect it to the phone network
• You want to port existing phone numbers to a VoIP setup
• You need multiple simultaneous call channels that scale with demand

You DON'T need a SIP trunk if:

• You use a cloud-based softphone platform like Softphone Plus — the trunk is already built into the service
• You have a hosted PBX where the provider manages everything

This is an important distinction. Many businesses over-engineer their phone setup by buying a PBX, a SIP trunk, and a softphone separately. A cloud softphone platform combines all three into one managed service.

How to Choose a SIP Trunk Provider

If you do need a standalone SIP trunk, evaluate based on:

Pricing Models

• Per-channel pricing — Pay for a fixed number of simultaneous calls (e.g., $15–$25/channel/month)
• Per-minute pricing — Pay for actual usage, better for low-volume callers
• Unlimited plans — Flat rate for unlimited domestic calling

Technical Requirements

• Codec support — G.711 is universal; G.729 saves bandwidth; Opus delivers the best quality
• SRTP support — Encrypted media for secure calls
• Failover — What happens if the primary route goes down?
• Number porting — Can you bring your existing numbers?

Reliability

• Uptime SLA — 99.99% is standard for quality trunk providers
• Geographic redundancy — Multiple POPs (Points of Presence) for reliability
• Support — 24/7 technical support for trunk issues

SIP Trunking Costs Breakdown

For a typical small business with 10 employees:

The cloud softphone model (like Softphone Plus) eliminates the PBX and trunk entirely — you're paying for one service that does everything.

The Simpler Alternative

SIP trunking makes sense if you're running an existing on-premise PBX and need to cut costs on your phone lines. But if you're starting fresh or ready to modernize, a cloud-based softphone platform is the simpler path:

• No PBX hardware to buy or maintain
• No SIP trunk to configure and manage
• No separate bills from three different vendors
• One dashboard for calling, recording, analytics, and team management

Try Softphone Plus free for 14 days — no PBX, no trunk, no hardware required.