How to Connect Phone Numbers to ANY Voice AI Agent (2026 SIP Trunking Guide)
Most businesses deploy voice AI agents only to hit a wall - customers can't call them from real phone numbers. SIP trunking solves this by bridging your AI to the public telephone network. This guide shows exactly how to configure Twilio SIP trunks with platforms like Vapi in , whether you're porting existing numbers or buying new ones.
What Is SIP Trunking for Voice AI?
Traditional phone systems route calls through physical copper lines - a technology unchanged for decades. SIP trunking replaces this with internet protocols, creating a digital bridge between your voice AI and the public telephone network (PSTN). Without SIP trunking, your AI agent can only operate within digital platforms like web browsers or apps.
The breakthrough comes from how SIP trunks handle protocol conversion. When a customer calls your AI's phone number, the SIP trunk converts the analog signal to digital data your AI can process. For outbound calls, it reverses this process - taking your AI's digital output and converting it to a format traditional phones understand.
Key stat: Businesses using SIP trunking with AI agents report 73% faster call resolution compared to human-only teams, while cutting telephony costs by 30-60%.
Step 1: Twilio SIP Trunk Configuration
Twilio's Elastic SIP Trunking provides the most reliable infrastructure for voice AI deployments. The setup involves four core components: creating the trunk, setting up credentials, configuring access control, and defining call routing.
Step 1.1: Create Your SIP Trunk
Log into your Twilio console and navigate to Elastic SIP Trunking. Click "Create New Trunk" and name it (e.g., "Vapi-Production"). The friendly name helps identify the trunk later if managing multiple environments.
Step 1.2: Set Up Credentials
Under "Credential Lists," create a new list with a secure username/password combination. These credentials will authenticate your voice AI platform when making outbound calls. Twilio requires passwords with uppercase, lowercase, numbers, and special characters.
Step 1.3: Configure Access Control
Navigate to "IP Access Control Lists" and create two lists:
- ACL1: 0.0.0.0/0 (allows all IPs - use only for testing)
- ACL2: Your voice AI platform's IP range (e.g., Vapi uses 128.0.0.0/16)
Pro tip: Always restrict SIP trunk access to your AI platform's specific IP ranges in production. Open ACLs (0.0.0.0) leave you vulnerable to toll fraud.
Step 2: Inbound vs Outbound Setup
Inbound and outbound calling require different SIP trunk configurations. Inbound calls (customers calling your AI) need proper number assignment and call routing. Outbound calls (AI calling customers) require authentication credentials.
Inbound Configuration
For inbound calls, purchase or port a phone number in Twilio and assign it to your SIP trunk. Then configure the "Origination URI" to point to your voice AI's SIP endpoint. This tells Twilio where to route incoming calls.
Outbound Configuration
Outbound calls require your voice AI platform to authenticate with the SIP trunk using the credentials created earlier. In Vapi, this involves entering the SIP trunk username, password, and Twilio's SIP domain in your agent settings.
Note: Most voice AI platforms handle SIP authentication automatically once you provide the trunk credentials. The complex SIP protocol negotiation happens behind the scenes.
Step 3: Vapi Integration
Vapi provides one of the simplest SIP trunk integrations for voice AI. After creating your Twilio SIP trunk, you'll need to configure three key settings in Vapi's dashboard.
Step 3.1: SIP Credentials
In your Vapi agent settings, navigate to the "Telephony" section. Select "SIP Trunk" as your connection method and enter:
- SIP Username (from your Twilio credential list)
- SIP Password
- Twilio SIP Domain (usually something like "pstn.twilio.com")
Step 3.2: Caller ID
Set your Twilio-purchased phone number as the outbound caller ID. This ensures customers see your business number when your AI calls them.
Step 3.3: Webhook URLs
Configure status callback URLs so Vapi receives call events from Twilio. These webhooks enable features like call recording analytics and real-time monitoring.
At the 8:45 mark in the video tutorial, you'll see exactly where these settings appear in Vapi's interface.
Step 4: Porting Existing Numbers
Porting your existing business number to Twilio maintains continuity while upgrading to AI-powered calling. The process takes 7-10 business days and requires documentation from your current provider.
Step 4.1: Check Portability
In Twilio's console, use the "Number Porting" tool to check if your number is eligible. Most US local and toll-free numbers can be ported, but some restrictions apply for numbers from certain carriers.
Step 4.2: Submit Port Request
You'll need to provide:
- Recent phone bill showing number ownership
- Account number and PIN from current provider
- Business verification documents
Step 4.3: Configure Post-Port Routing
Once ported, assign the number to your SIP trunk and configure call routing just like a newly purchased number. Test inbound/outbound calls thoroughly as some carrier settings may differ.
Warning: Don't cancel service with your old provider until the port completes. Doing so may release your number back into the pool, making it unavailable.
Security Considerations
SIP trunking introduces new security considerations for voice AI deployments. Implement these best practices to prevent toll fraud and unauthorized access.
IP Restriction
As mentioned earlier, always restrict SIP trunk access to your voice AI platform's specific IP ranges. Open ACLs (0.0.0.0/0) are only for testing.
Encryption
Enable TLS for SIP signaling and SRTP for media encryption. This prevents eavesdropping on calls between your AI and customers.
Rate Limiting
Configure call rate limits in Twilio to prevent abuse. A typical business might set limits of 10 concurrent calls and 500 calls/day per number.
Monitoring
Set up Twilio alerts for unusual call patterns. Sudden spikes in international calls or concurrent sessions may indicate fraud.
Cost Analysis
SIP trunking costs fall into three categories: trunk fees, calling rates, and number costs. Here's the pricing breakdown for US-based deployments.
Base Costs
- SIP Trunk: $0.50/month per trunk (most businesses need just one)
- Phone Numbers: $1/month per local number, $2/month per toll-free
- Number Porting: $15 one-time fee per number
Usage Costs
- Inbound Calls: $0.0045/minute in US
- Outbound Calls: $0.013/minute to US numbers
- International: Varies by country (see Twilio's rate deck)
Cost saver: Businesses handling 5,000+ minutes/month qualify for volume discounts. Contact Twilio sales for negotiated rates.
Watch the Full Tutorial
For visual learners, our video tutorial walks through every step of the SIP trunking setup process. At 12:30, you'll see the exact moment we test the first AI-powered call through the SIP trunk - a breakthrough moment for any business implementing voice AI.
Key Takeaways
SIP trunking transforms voice AI from a digital-only tool to a full telephone system replacement. The technical setup may seem daunting, but platforms like Twilio and Vapi have simplified the process dramatically.
In summary: Create your Twilio SIP trunk, configure inbound/outbound settings, integrate with your voice AI platform, and port existing numbers if needed. The result? An AI agent that handles calls indistinguishably from human staff, available 24/7 at a fraction of the cost.
Frequently Asked Questions
Common questions about SIP trunking for voice AI
SIP trunking connects your voice AI to the public telephone network (PSTN) so customers can call your AI agent from any phone number. Without SIP trunking, your AI can only make/receive calls within digital platforms.
SIP trunks handle the conversion between internet protocols and traditional phone systems. They translate the digital signals your AI understands into analog audio for regular phones, and vice versa.
- Enables calling to/from any phone number worldwide
- Maintains your existing business phone numbers
- Provides better call quality than VoIP alternatives
Twilio SIP trunking costs $0.0045 per minute for inbound/outbound calls in the US, plus $1/month per phone number. Porting existing numbers costs $15 per number. Most businesses spend $20-$200/month depending on call volume.
International calling rates vary by destination. For example, calls to UK landlines cost $0.011/minute, while calls to Australian mobiles cost $0.14/minute. Always check Twilio's latest rate deck for your target markets.
- Base cost: $0.50/month per SIP trunk
- Local numbers: $1/month
- Toll-free numbers: $2/month
Yes, you can port most existing business numbers to Twilio for use with voice AI. The porting process takes 7-10 business days and requires authorization from your current provider.
Some restrictions apply for toll-free and international numbers. Numbers from certain rural carriers or resellers may have additional porting requirements. Twilio's porting team can verify eligibility before you submit the request.
- Keep service active with current provider during port
- Have a recent bill showing number ownership ready
- Prepare for 1-2 hours of downtime when port completes
Inbound SIP trunking routes calls to your AI when customers dial your number. This requires purchasing/porting a phone number and configuring call routing in Twilio.
Outbound SIP trunking lets your AI initiate calls to customers. This requires setting up authentication credentials that your voice AI platform uses to authenticate with Twilio's SIP servers.
- Inbound: Number + call routing configuration
- Outbound: Username/password authentication
- Most businesses configure both for full functionality
Twilio SIP trunks use enterprise-grade security including TLS encryption, IP allowlisting, and authentication. For maximum security, configure Access Control Lists (ACLs) to restrict which IPs can connect to your SIP trunk.
Enable SRTP (Secure Real-Time Transport Protocol) for media encryption. This prevents eavesdropping on call audio. Also implement call rate limits to prevent toll fraud attempts.
- TLS 1.2+ for signaling encryption
- SRTP for media encryption
- IP allowlisting prevents unauthorized access
Most modern voice AI platforms support SIP trunking including Vapi, Retell AI, Bland AI, and Voiceflow. The setup process is similar across platforms - you'll need to configure your SIP trunk credentials in the AI platform's settings.
Some platforms like Vapi provide pre-built Twilio integrations that simplify the setup. Others may require manual SIP configuration. Check your platform's documentation for specific SIP trunking requirements.
- Vapi: Native Twilio integration
- Retell AI: Manual SIP configuration
- Bland AI: Both options available
Yes, Twilio supports SIP trunking in over 100 countries. International call rates vary by destination. Some countries require local phone number registration. For global deployments, consider purchasing local numbers in each target market.
Certain countries have regulatory requirements for AI-powered calling. For example, some EU nations require explicit consent for automated calls. Always check local telecommunications laws before deploying internationally.
- Check Twilio's international coverage list
- Purchase local numbers for better answer rates
- Be aware of time zone differences in routing
GrowwStacks handles the complete SIP trunking setup including number porting, Twilio configuration, and AI platform integration. We ensure proper security settings, call routing, and failover systems.
Our team can deploy a production-ready SIP trunk for your voice AI in under 48 hours. We'll handle the technical complexities while you focus on designing conversational flows for your AI agent.
- Fast deployment: 48-hour turnaround for most setups
- Number porting assistance
- Ongoing monitoring and optimization
Ready to Make Your AI Agent Callable From Any Phone?
Every day without SIP trunking means missed customer calls and manual call handling costs. Our team can have your AI answering calls on your business number within 48 hours.