SIP Trunk — Private Call Routing for Enterprise Voice AI
Connect your existing telephony infrastructure directly to VoiceB.ai's AI agents through a dedicated, fully managed SIP trunk — no number porting, no middleware, no compromise.
Overview
The SIP Trunk feature in VoiceB.ai allows enterprise customers to route calls from their existing PBX or Session Border Controller (SBC) directly into VoiceB.ai's AI agents over a private, dedicated SIP trunk. Instead of relying on shared telephony infrastructure or porting phone numbers to a third-party carrier, organizations keep full control of their telecom stack while seamlessly connecting it to conversational AI.
When a call arrives at your PBX, your SBC forwards it via SIP INVITE to VoiceB.ai's termination URI. VoiceB.ai processes the incoming SIP headers, optionally calls a pre-call webhook to pull real-time CRM context, applies routing rules to select the right AI agent, and then bridges the caller into the conversation — all within milliseconds.
The feature is accessible from the main navigation sidebar under SIP Trunk.
The SIP Trunk dashboard showing trunk metadata (name, SID, termination URI, region, security posture) and the IP Access Control list for whitelisting client SBC addresses.
How It Works
The call flow is straightforward:
Caller dials your business number — The number stays on your carrier; nothing changes for the end user.
Your PBX/SBC forwards the call — Via SIP INVITE to the VoiceB.ai termination URI (e.g.,
vidaone-espana-…-voiceb-v-spain.pstn.twilio.com), including any custom SIP headers your system attaches.VoiceB.ai processes the call — It captures configured SIP headers, fires the pre-call webhook to fetch dynamic context from your CRM or backend, evaluates routing rules, and selects the appropriate AI agent.
AI agent handles the conversation — The agent uses the injected context (caller identity, account data, lead info) to personalize the dialogue in real time.
Transfer back to your PBX — When the agent needs to hand off to a human, VoiceB.ai fires a pre-transfer webhook with conversation data and routes the call to your SBC address.
Configuration Guide
The SIP Trunk settings page is organized into clearly separated configuration sections, accessible via the left-hand tab navigation.
1. Trunk Information
This read-only panel displays the core identifiers for your trunk:
Trunk Name
Human-readable identifier (e.g., voiceb-v-spain)
Trunk SID
Unique system ID for API references
Termination URI
The SIP endpoint your SBC should send INVITE requests to
Security
Active security mechanisms (TLS, IP ACL)
Region
Data-processing region (e.g., Dublin IE1)
2. Security
This section manages two layers of protection:
IP Access Control — Whitelist the IP addresses of your SBC so only authorized sources can send SIP traffic to the trunk. Each entry has a label and an IP address or CIDR range.
SIP Credentials — Optionally configure SIP digest authentication credentials. When enabled, clients must authenticate with username/password in addition to IP whitelisting, adding a second factor of security.
IP Access Control showing two whitelisted Vodafone SBC addresses, plus the SIP Credentials section for optional digest authentication.
3. Twilio Trunk Configuration
These settings are read from and saved directly to the underlying Twilio infrastructure:
Secure Trunking (SRTP + TLS)
Toggle on/off — encrypts both media and signaling
Call Transfer
Disable all · Enable all · SIP only
Call Recording
Do not record · Record from ringing · Record from answer · Dual-channel variants
Recording Trim
Do not trim · Trim silence
Disaster Recovery URL
Fallback TwiML URL if the primary endpoint becomes unreachable
4. Client Integration
Configure how VoiceB.ai communicates back to your infrastructure during call transfers:
Client SBC Address — The FQDN or IP (plus port) of your PBX/SBC where transferred calls should be routed.
Pre-Transfer Webhook URL — A URL called with full conversation data (transcript, intent, extracted entities) before each transfer, so your CRM can prepare the receiving human agent.
Webhook Signing Secret — An HMAC-SHA256 secret for verifying webhook payload authenticity via the x-webhook-signature header.
Client Integration configuration with SBC address, pre-transfer webhook URL, and HMAC signing secret.
5. Pre-Call Webhook
This is one of VoiceB.ai's most powerful differentiators. On every incoming call — before the AI agent even picks up — VoiceB.ai fires a webhook to your backend system.
Pre-Call Webhook URL — The endpoint VoiceB.ai calls (POST or GET) on each inbound call.
Pre-Call Payload Template — A JSON template using {{placeholder}} tokens that map to SIP headers and call metadata. For example:
This lets you inject dynamic CRM data, customer tier, open support tickets, or any other context into the AI agent's prompt — making each conversation hyper-personalized from the first second.
6. Phone Numbers (DDIs)
Assign phone numbers (DDIs) to the trunk. Assigned numbers are routed through the custom SIP trunk instead of native provider routing.
The Linked Agents table shows which AI agents are associated with this trunk and whether they use the webhook-based transfer tool. A simple toggle lets you link or unlink agents instantly.
7. SIP Headers
Configure which SIP headers to capture on inbound calls and inject on transfers. Each header is configured with:
Header Name
The SIP header name (e.g., X-Original-Caller)
Direction
INBOUND, OUTBOUND, or BOTH
Behavior
ROUTING (used in routing rules) or PASSTHROUGH (forwarded to webhooks)
Required
Whether the header must be present
Forward to
Where the header value is sent (Webhooks, Agent, etc.)
The Caller ID Header selector lets you pick which SIP header carries the real caller number, replacing the SIP From field throughout the system (in the agent, in leads, in logs).
8. Routing Rules
Define rules to route incoming calls to specific agents based on SIP header values, webhook response fields, or caller ID patterns. Rules are evaluated top-to-bottom with first-match-wins logic. A Default Agent is configured as a fallback when no rule matches.
9. Call Tester
A built-in tool to validate your configuration without placing a real call, or to make an actual test call through the full pipeline:
Validate mode — Enter a DDI and simulated SIP headers, then click Validate to dry-run the routing logic and confirm the correct agent is selected.
Test Call mode — Enter your personal phone number and VoiceB.ai will call you, bridging you through the full incoming-call pipeline (SIP headers → pre-call webhook → routing rules → AI agent) so you can experience the end-to-end flow.
Use Cases
Enterprise Contact Center Augmentation
Large contact centers (like the Vodafone España example shown in the interface) can offload frontline call handling to AI agents while keeping their existing carrier contracts, PBX infrastructure, and phone numbers untouched. The AI handles qualification, routing, FAQ resolution, and data collection — then transfers to human agents with full context via the pre-transfer webhook.
Dynamic CRM-Driven Conversations
Using the pre-call webhook, a financial services company can look up the caller's account, check their portfolio status, and inject personalized data into the AI agent's prompt before the caller even hears "hello." This eliminates the generic IVR experience.
Multi-Agent Routing by Business Unit
A multinational can use SIP header-based routing rules to direct calls to different AI agents depending on the department, product line, or language — all on a single trunk, with no changes to the PBX routing logic.
Gradual AI Adoption
Organizations can start by routing a single DDI to an AI agent, test performance, and progressively add more numbers and agents without disrupting existing telephony workflows.
Competitive Advantages vs. Other AI Voice Platforms
VoiceB.ai vs. Bland AI
Bland AI offers a comprehensive SIP integration with a setup wizard, auto-discovery, failover, and number porting. However, the approach differs significantly:
Architecture philosophy — Bland expects you to either route calls to their infrastructure or port your numbers entirely. VoiceB.ai's model is the opposite: your PBX stays in control, and VoiceB.ai acts as an intelligent termination point. This is crucial for regulated enterprises that cannot port numbers or relinquish telephony control.
Pre-call webhook with templated payloads — VoiceB.ai's pre-call webhook lets you define a JSON payload template with {{placeholder}} tokens mapped to SIP headers. This means the AI agent has full CRM context before the conversation starts. Bland's approach focuses on pathway and persona configuration per number but doesn't offer the same depth of real-time, header-driven context injection.
SIP header management — VoiceB.ai provides a granular SIP header configuration table with direction (inbound/outbound/both), behavior (routing vs. passthrough), required/optional flags, and forwarding targets. This level of header control is essential for enterprise PBX integrations where headers carry call metadata like original caller ID, lead IDs, and interaction IDs. Bland supports custom headers but with less configurability.
Pre-transfer webhook — Before handing off to a human agent, VoiceB.ai fires a webhook with conversation data and supports HMAC-SHA256 signature verification for security. This allows the receiving agent's CRM screen to be pre-populated. Bland relies on standard call transfer without this middleware step.
No-code UI — VoiceB.ai's entire configuration is done through a visual dashboard with no API calls required. Bland's wizard is also user-friendly, but many advanced features (failover, codecs, alerts) require API calls.
VoiceB.ai vs. ElevenLabs SIP Trunking
ElevenLabs is primarily a voice synthesis platform that added SIP trunking to enable its AI agents to handle phone calls. The SIP integration is functional but comparatively basic:
Configuration depth — ElevenLabs' SIP setup covers transport type, media encryption, outbound address, optional custom headers, and optional digest auth. That's the full extent. VoiceB.ai offers all of this plus IP ACL management, pre-call webhooks with templated payloads, pre-transfer webhooks with HMAC signing, SIP header routing behavior configuration, call recording controls, disaster recovery URLs, routing rules, and a built-in call tester.
Routing intelligence — ElevenLabs assigns one agent per phone number and suggests using your PBX for conditional routing. VoiceB.ai has a native routing rules engine that evaluates SIP headers, webhook response fields, and caller ID patterns to dynamically select agents — moving routing intelligence from the PBX into the AI layer.
Enterprise security — VoiceB.ai provides dual-layer security (IP ACL + SIP digest auth), HMAC-SHA256 webhook signing, and configurable TLS/SRTP — all from the dashboard. ElevenLabs supports digest auth or IP-based ACL, and notes that for IP whitelisting you need to contact their support team. Their static IP infrastructure requires whitelisting an entire /24 block, which is less precise.
Call testing — VoiceB.ai includes a built-in Call Tester with both a dry-run validation mode (to verify routing logic without placing a call) and a live test call mode. ElevenLabs' troubleshooting guidance is documentation-based rather than tooling-based.
Transfer workflow — VoiceB.ai's Client Integration section with pre-transfer webhooks, SBC address configuration, and conversation data forwarding creates a complete transfer workflow. ElevenLabs does not offer an equivalent pre-transfer data handoff mechanism.
Summary Comparison Table
Dedicated SIP trunk per customer
✅
✅
✅
IP Access Control (ACL)
✅ Dashboard
✅ API/Wizard
⚠️ Contact support
SIP digest authentication
✅
✅
✅
TLS + SRTP encryption
✅ Toggle
✅ Required
✅ Configurable
Pre-call webhook (CRM context)
✅ With templated payloads
❌
❌
SIP header capture & routing
✅ Full config table
⚠️ Basic
⚠️ Custom headers only
Routing rules engine
✅ Header/webhook/caller-based
❌ Per-number only
❌ Per-number only
Pre-transfer webhook
✅ With HMAC signing
❌
❌
Client SBC address config
✅
❌
✅ (outbound address)
Call recording controls
✅ Multiple modes
⚠️ Platform-level
⚠️ Additional config
Disaster recovery / failover
✅ TwiML fallback URL
✅ Multi-server failover
❌
Built-in call tester
✅ Validate + live test
✅ SIP trace viewer
❌
Number porting
Not required (BYO trunk)
✅ Full porting support
Not required
No-code setup
✅ Full dashboard
⚠️ Wizard + API for advanced
✅ Basic dashboard
Agent linking / multi-agent
✅ Per-trunk linking
✅ Per-number pathway
✅ Per-number assignment
Getting Started
Navigate to SIP Trunk in the VoiceB.ai sidebar.
Review your Trunk Information — note the Termination URI; this is what your SBC will send SIP INVITEs to.
Configure Security — add your SBC's IP addresses to the ACL.
Set up the Twilio Trunk Configuration — enable Secure Trunking, configure recording preferences.
Enter your Client Integration details — SBC address for transfers and the pre-transfer webhook URL.
Configure a Pre-Call Webhook — point it at your CRM endpoint and define the payload template.
Assign Phone Numbers (DDIs) and link your AI agents.
Define SIP Headers to capture and forward.
Set up Routing Rules if you need conditional agent selection.
Use the Call Tester to validate everything end-to-end.
The SIP Trunk feature is available for enterprise customers. For provisioning, contact your VoiceB.ai account representative or reach out at https://voiceb.ai/solutions/appointment-booking
Last updated
Was this helpful?