Looking to modernise your company phone system without replacing all your hardware? A SIP Trunk is often the simplest and most cost-effective answer. This guide explains, without unnecessary jargon, what a SIP Trunk is, how it works, how much it costs and how to pick the right provider.
What is a SIP Trunk?
A SIP Trunk is a telephone link that connects your existing PBX, IP-PBX or softphone platform to the public telephone network over the Internet, instead of legacy analogue or ISDN lines.
In practice, it replaces your old copper lines (being switched off across Europe) and lets you make and receive calls through a simple Internet connection, while keeping your numbers and your equipment.
How does a SIP Trunk work?
The principle is simple:
- Your phone system (IP-PBX, 3CX, Asterisk, FreePBX, etc.) connects to your provider's SIP Trunk.
- Calls travel as Voice over IP between your company and the public network.
- You manage your channels (simultaneous calls) and numbers from a single interface.
Two key notions:
- Channel: one simultaneous call. A 5-channel trunk allows 5 calls at once.
- Number (DID): your phone number. A single trunk can carry several numbers, in France and abroad.
SIP Trunk or Hosted PBX (Centrex): what's the difference?
It's the most common question, and the answer depends on your setup:
- A SIP Trunk suits companies that already own a phone system and simply want to connect it to the network. You keep control of your installation.
- A Hosted PBX (Centrex) is a fully cloud-hosted phone system: nothing to manage on site. Ideal if you're starting from scratch.
In short: you already have a PBX → SIP Trunk; you want a turnkey solution → Hosted PBX.
How much does a SIP Trunk cost?
Pricing depends on three factors:
- Number of channels (simultaneous calls): the main price driver.
- Numbers included and any extra numbers.
- Usage: bundled minutes or per-minute billing by destination.
As a benchmark, a professional SIP Trunk starts around €14 excl. VAT per month for a small business and scales with the number of channels. At ODOIP, every Trunk plan includes one number and the channels you need, with calls to France and international destinations.
How to choose your SIP Trunk: the checklist
Before you sign, check these points:
- Compatibility: does the trunk work with your IP-PBX (3CX, Asterisk, FreePBX, Yeastar...)?
- Number of channels: plan roughly one channel per 3-4 active users.
- Number portability: can you keep your current numbers?
- Quality and monitoring: does the provider host in France with responsive support?
- Security: is the trunk protected against fraud (risky-destination blocking, authentication)?
- Transparent pricing: are call rates clear, with no surprises?
Why choose ODOIP for your SIP Trunk
ODOIP is a French electronic communications operator, with its own servers hosted in France and an engineering team that truly answers. You get French and international numbers, 5 to 50 channels, monitoring and responsive support, and guided setup compatible with most PBXs.
Discover our SIP Trunk plans, or compare with our Hosted PBX solution if you prefer a turnkey system.
FAQ — SIP Trunk
Does a SIP Trunk really replace ISDN/PSTN lines? Yes. With the progressive shutdown of legacy copper networks, the SIP Trunk is the standard replacement to connect your telephony to the public network.
Can I keep my current numbers? Yes, number portability is available in the vast majority of cases.
How many channels do I need? Roughly one channel per 3-4 users who make calls regularly. A 15-person company is often comfortable with 5 channels.