SIP trunking is a method of connecting a business phone system to the public telephone network using the internet instead of traditional copper phone lines. SIP stands for Session Initiation Protocol, which is the standard protocol for setting up, managing, and tearing down voice calls over the internet. A “trunk” is the connection between your phone system and the outside world.
In plain terms: if your office has a PBX (the box that manages all your desk phones), SIP trunking replaces the physical phone lines going into that box with an internet connection. Calls still work the same way from the user’s perspective, but they travel over the internet instead of dedicated phone circuits.
How SIP Trunking Works
A traditional office phone setup looks like this: your PBX connects to the phone company through physical lines (ISDN, PRI, or analog). Each line handles one or two calls at a time. You pay per line and often per minute for calls.
With SIP trunking, the physical lines are replaced:
- Your PBX connects to a SIP trunk provider through your internet connection. This requires a SIP-compatible PBX (most modern ones support it, and older ones can be upgraded).
- When someone in your office makes a call, the PBX sends the call over the internet to the SIP trunk provider.
- The provider routes the call to the public telephone network (PSTN), reaching landlines, mobiles, and international numbers just like a traditional call.
- Incoming calls work in reverse. The provider receives the call from the PSTN and delivers it to your PBX over the internet.
The caller on the other end cannot tell the difference. The call quality, reliability, and phone number all stay the same.
Why Businesses Use SIP Trunking
Cost savings
Traditional phone lines charge a monthly fee per line and per-minute rates for calls. SIP trunking consolidates your phone connectivity into your existing internet connection and typically charges lower per-minute rates. Businesses switching from ISDN to SIP trunking commonly save 30 to 50% on their phone bills.
Flexible capacity
With physical phone lines, you pay for a fixed number of lines whether you use them or not. SIP trunking scales with demand. Need ten concurrent calls during a product launch? No problem. Drop back to three calls on quiet days? You only pay for what you use (with most providers).
Keep your existing equipment
If you have invested in a PBX and desk phones, SIP trunking lets you keep using them. You replace the phone lines, not the entire system. This makes it a practical upgrade path for businesses that are not ready to move to a fully cloud-based phone system.
Number portability
You can port your existing phone numbers to a SIP trunk provider. Your customers keep dialing the same number, and nothing changes on their end.
Geographic flexibility
SIP trunks are not tied to a physical location. You can have phone numbers for multiple cities or countries, all terminating at the same PBX. This is useful for businesses with regional numbers but a centralized office.
SIP Trunking vs. Hosted VoIP
These are two different ways to get your phone calls onto the internet.
| Feature | SIP Trunking | Hosted VoIP |
|---|---|---|
| You need | A PBX (hardware or software) | Just phones or an app |
| Who manages the system | You (or your IT team) | The VoIP provider |
| Customization | High (full control over PBX) | Limited to provider’s features |
| Setup complexity | Moderate | Simple |
| Best for | Businesses with an existing PBX | Businesses starting fresh or small teams |
| Cost structure | Per-channel or per-minute | Per-user per month |
If you already have a PBX and want to cut costs on phone lines, SIP trunking is the right choice. If you are starting from scratch and do not want to manage a phone system, hosted VoIP is simpler.
Common SIP Trunking Providers
In Germany and Europe:
- Sipgate: Well-known German provider with SIP trunk plans for businesses of all sizes
- Easybell: Budget-friendly option with straightforward SIP trunk pricing
- Placetel (Cisco): GDPR-compliant, good integration options
- Deutsche Telekom (Company Flex): Traditional carrier offering SIP trunk alongside their network
In the US:
- Twilio: Developer-friendly, pay-per-minute SIP trunking
- Vonage: Business SIP trunking with CRM integrations
- Bandwidth: Carrier-grade SIP trunking at competitive rates
Setting Up SIP Trunking
- Check your PBX compatibility. Your PBX needs to support SIP. Most modern systems do. Older hardware may need a firmware update or a SIP gateway adapter.
- Assess your internet connection. Each concurrent call needs about 80 to 100 kbps of bandwidth in each direction. For 10 simultaneous calls, you need roughly 1 Mbps dedicated to voice. A standard business internet connection handles this easily.
- Choose a SIP trunk provider. Compare pricing (per-channel vs. per-minute), features, and geographic coverage.
- Configure your PBX. Enter the SIP trunk credentials (server address, username, password, codec settings) into your PBX admin panel.
- Port your numbers. If you want to keep your existing phone numbers, initiate the porting process with your new provider. This usually takes one to two weeks.
- Test. Make inbound and outbound calls. Check call quality, verify that caller ID displays correctly, and confirm that features like call forwarding and voicemail still work.
SIP Trunking and AI Phone Assistants
SIP trunking and AI phone assistants work well together. Your PBX’s call routing rules can direct unanswered or overflow calls to an AI assistant like Safina.
The setup is straightforward: add Safina’s phone number as a forwarding destination in your PBX. When a call goes unanswered or the queue is full, the PBX routes the call over the SIP trunk to Safina. The AI handles the conversation and delivers a summary to you.
This combination gives you the control of a PBX with the reliability of an AI assistant that never misses a call.
Related Terms
- VoIP: The broader technology that SIP trunking is built on
- Business Phone Number: The numbers you connect through your SIP trunk
- Call Routing: How your PBX directs calls, including to AI assistants via SIP
- IVR: Phone menu systems that run on your PBX and work over SIP trunks