What Is SIP Trunking?

SIP trunking is a method of delivering phone service over the internet using the Session Initiation Protocol (SIP). It replaces traditional analog phone lines (PRI or POTS) by connecting your existing PBX phone system to the public telephone network through your internet connection.

Think of a SIP trunk as a virtual phone line. Instead of physical copper wires running into your building, your calls travel over your broadband connection — reducing costs and increasing flexibility.

How SIP Trunking Works

SIP trunking bridges your on-premises phone system with VoIP infrastructure:

1. Your PBX handles internal calls and call management as it normally would.
2. Outbound calls are converted from your PBX's format into SIP (a VoIP signaling protocol) by the SIP trunk.
3. The call travels over the internet to your SIP trunking provider's network.
4. The provider connects the call to the public telephone network (PSTN) so it reaches any phone number.
5. Inbound calls reverse the process — arriving from the PSTN, converting to SIP, and reaching your PBX.

SIP trunks are sold in channels, where each channel supports one concurrent call. A business that needs up to 10 simultaneous calls would provision a 10-channel SIP trunk.

Why SIP Trunking Matters for Business

SIP trunking delivers measurable benefits over traditional phone circuits:

• 40–60% cost reduction compared to PRI lines, with lower per-minute rates and no long-distance surcharges.
• Scalability on demand — add or remove channels instantly instead of ordering new physical lines.
• Business continuity — SIP trunks can failover to backup locations or mobile numbers automatically during outages.
• Number portability — keep your existing business phone numbers when switching providers.
• Unified communications — SIP supports voice, video, and messaging over a single connection.

For businesses with an existing PBX investment, SIP trunking is the most cost-effective path to modern phone capabilities without replacing hardware.

SIP Trunking vs. PRI Lines

PRI (Primary Rate Interface) is the traditional way to connect a PBX to the phone network:

• PRI delivers 23 voice channels over a dedicated physical circuit. Adding capacity means ordering additional circuits with lead times of weeks.
• SIP trunking delivers channels virtually over your internet connection. Adding capacity is a configuration change that takes minutes.
• Cost — PRI lines typically run $300–$500 per month for 23 channels. SIP trunking delivers the same capacity for $100–$200.
• Flexibility — SIP channels can be allocated across multiple locations; PRI circuits are tied to a physical address.

How AI Is Changing SIP Trunking

SIP trunking connects your phone system to the world. AI transforms what happens once calls arrive:

• AI phone agents sit behind SIP trunks, answering inbound calls with natural conversation instead of rigid IVR menus.
• Intelligent routing uses AI to analyze caller intent in real time and direct calls to the right resource.
• Automated handling — AI resolves routine calls (scheduling, FAQs, lead capture) without involving staff.

Sawy works with SIP-enabled phone systems, letting businesses add an AI agent to their existing infrastructure. Calls arrive through your SIP trunk, the AI handles them, and human agents only engage when their expertise is needed.

FAQ

Do I need a PBX to use SIP trunking?

Yes, traditional SIP trunking requires an IP-PBX. However, if you don't have a PBX, a hosted VoIP or cloud phone system provides the same benefits without on-premises equipment.

How much bandwidth does SIP trunking require?

Each concurrent call uses approximately 80–100 Kbps of bandwidth. A 10-channel SIP trunk needs about 1 Mbps of dedicated bandwidth for reliable call quality.

Can I keep my phone numbers when switching to SIP?

Yes. Number porting is standard with SIP trunking providers. The process typically takes 1–3 weeks depending on your current carrier.