How Carrier Lookup Blocks Your AI Agent's Phone Number

Your AI agent needs to verify a phone number. You provision one from Twilio, Vonage, or any virtual number provider. The verification code never arrives. No error, no explanation — just silence.

The reason: carrier lookup. Every major service checks your number's line type before sending an OTP. If it returns voip, the SMS is never dispatched.

What Is Carrier Lookup?

When a service like Stripe or WhatsApp receives a phone number for verification, they query a carrier lookup API. This API checks telecom databases (LERG, NPAC, HLR) and returns metadata about the number:

{
  "phone_number": "+15551234567",
  "line_type_intelligence": {
    "type": "nonFixedVoip",
    "carrier_name": "Twilio Inc.",
    "mobile_country_code": null,
    "mobile_network_code": null
  }
}

That nonFixedVoip classification is the kill switch. The service sees it and silently drops your verification request.

Which Services Block VoIP?

The pattern is consistent: VoIP = blocked. Mobile = accepted.

How to Check Your Number

Before using any number with your AI agent, run a carrier lookup yourself. Here's how with the Twilio Lookup API:

from twilio.rest import Client
 
client = Client("ACCOUNT_SID", "AUTH_TOKEN")
 
result = client.lookups.v2.phone_numbers("+15551234567").fetch(
    fields="line_type_intelligence"
)
 
lti = result.line_type_intelligence
print(f"Type: {lti['type']}")           # "mobile" = good, "nonFixedVoip" = blocked
print(f"Carrier: {lti['carrier_name']}") # Who owns the number

What the types mean:

Why Twilio Numbers Always Fail

Twilio owns its number ranges. The LERG (Local Exchange Routing Guide) database permanently maps those ranges to Twilio's OCN (Operating Company Number). No matter what you do, a carrier lookup on a Twilio number will always return:

carrier_name: "Twilio"
type: "nonFixedVoip"

This isn't a bug — it's how the telecom system classifies numbers. Twilio numbers are genuinely VoIP. The same applies to Vonage, Bandwidth, and every other virtual number provider.

The Fix: Real SIM Infrastructure

The only way to pass carrier lookup is to use numbers provisioned on real SIM infrastructure — numbers that are registered on cellular networks with actual IMSI values and carrier connections.

import agentsim
 
async with agentsim.provision(agent_id="my-agent") as session:
    print(f"Number: {session.number}")
    # Carrier lookup returns: type: "mobile", carrier: "T-Mobile USA"
    
    # Use session.number for verification...
    otp = await session.wait_for_otp(timeout=120)
    print(f"Code: {otp.code}")

When you run a Twilio Lookup on an AgentSIM number:

{
  "line_type_intelligence": {
    "type": "mobile",
    "carrier_name": "T-Mobile USA"
  }
}

The difference is at the infrastructure level. AgentSIM provisions on real SIM cards connected to cellular networks. The number genuinely is mobile — it's not a software trick or API wrapper.

Important: Carrier Lookup ≠ Guaranteed Acceptance

Passing carrier lookup is necessary but not always sufficient. Some services apply additional checks:

• Behavioral analysis — Account creation velocity, IP reputation, browser fingerprinting
• Number reputation — Some number ranges get flagged from overuse
• Geographic consistency — IP location vs. phone number area code
• HLR queries — Real-time checks on whether the SIM is actively registered on a network

Carrier lookup returning mobile gets you past the first gate. The other gates depend on the specific service and how you're using the number.

Cost Comparison

Free tier: 10 sessions/month. No subscription, no idle numbers.

Try It

pip install agentsim-sdk

import agentsim
 
agentsim.api_key = "your-api-key"
 
async with agentsim.provision(agent_id="carrier-test") as session:
    print(f"Number: {session.number}")
    # Now use this number for verification
    otp = await session.wait_for_otp(timeout=120)
    print(f"OTP: {otp.code}")

Get an API key at agentsim.dev.

AgentSIM provisions real SIM-backed mobile numbers for AI agent verification. Carrier lookup returns mobile, not voip.