What is a SIP Trunk?
A SIP trunk (Session Initiation Protocol trunk) is a virtual phone line that connects your phone system to the public telephone network (PSTN) over the internet. Instead of physical copper wires, voice calls are transmitted as data packets over your existing internet connection.| Traditional Phone Line | SIP Trunk |
|---|---|
| Physical copper wires | Virtual connection over internet |
| Fixed capacity (23 channels per PRI) | Scalable on demand |
| Local provider lock-in | Provider-agnostic, works globally |
| High fixed monthly costs | Pay-per-use or wholesale rates |
How Does SIP Trunking Work?
Two protocols work together: SIP handles the signaling (setting up and tearing down calls), and RTP carries the actual voice audio.The Call Flow
Call Initiation
A SIP INVITE is sent to the provider’s gateway with the destination number and codec preferences (e.g., G.711 ulaw).
Authentication and Routing
The provider verifies the request via IP-based auth or username/password (digest auth), then routes the call to the PSTN.
Media Negotiation (SDP)
Both sides exchange SDP messages to agree on audio codecs, IP addresses, and ports for the voice stream.
Voice Streaming (RTP)
Voice audio flows as RTP packets between the endpoints, encoded using the agreed codec (typically G.711 ulaw at 64 kbps).
Key Terms
| Term | Meaning |
|---|---|
| Origination | Receiving inbound calls from the PSTN and delivering them to your system |
| Termination | Sending outbound calls from your system to the PSTN via the trunk |
| DID Number | A phone number assigned to your trunk that external callers can dial |
| Gateway | The provider’s server address where SIP traffic is sent |
| Codec | Audio encoding format (G.711 ulaw is the standard) |
How Bolna Accommodates SIP Trunking
Bolna’s Bring Your Own Telephony (BYOT) feature lets you connect any standards-compliant SIP trunk. Bolna runs an Asterisk-based SIP media server with the PJSIP channel driver. When you register your trunk:- Bolna creates a SIP endpoint with your gateway address, authentication details, and codec preferences
- Inbound calls arrive at Bolna’s server (
sip:13.200.45.61:5060), get matched to a registered DID, and route to the assigned AI agent - Outbound calls are sent through your trunk’s gateway using your credentials and caller ID
- The AI agent handles the conversation in real-time (speech-to-text, LLM response, text-to-speech)
Supported Authentication
| Method | How It Works | Best For |
|---|---|---|
Username/Password (userpass) | SIP digest auth with username and password | Zadarma, Vonage, DIDWW |
IP-Based (ip-based) | Bolna identified by source IP 13.200.45.61 | Plivo Zentrunk, Telnyx |
Benefits of Using a SIP Trunk with Bolna
Use Your Existing Telephony Relationships
Use Your Existing Telephony Relationships
Keep your current SIP provider, contracts, and phone numbers. Connect your trunk to Bolna and your AI agents are ready instantly.
Full Cost Control
Full Cost Control
Pay your SIP provider directly for call minutes at your own negotiated rates. Bolna charges separately only for AI processing.
Number Portability Without Migration
Number Portability Without Migration
Your DID numbers stay with your current provider. No porting, no downtime. Just point them to Bolna’s SIP server.
Provider Flexibility and Redundancy
Provider Flexibility and Redundancy
Switch providers without changing anything on Bolna. Configure multiple trunks from different providers for failover.
Global Coverage with Local Numbers
Global Coverage with Local Numbers
Bring US toll-free, Indian DIDs, European local numbers, or any numbers from your provider’s inventory.
Compliance and Regulatory Control
Compliance and Regulatory Control
Your existing compliance setup (DLT registration, STIR/SHAKEN) stays with your provider. Bolna does not interfere.
Elastic Scalability
Elastic Scalability
Scale from 1 concurrent call to thousands without hardware changes. Bolna scales alongside your trunk capacity.
Get Started
Follow these guides in order to set up SIP trunking with Bolna:Set Up a Plivo Trunk
Create inbound and outbound SIP trunks on Plivo Zentrunk
Set Up a Twilio Trunk
Create an Elastic SIP Trunk on Twilio
Register Trunk on Bolna
Register your trunk and add phone numbers via API
Receive Inbound Calls
Route incoming calls to Bolna AI agents
Make Outbound Calls
Place outbound calls from your trunk numbers
SIP Trunk API Reference
Full API reference for managing trunks and numbers