The Problem with VoIP Numbers
When developers first need to give an agent a phone number, the natural instinct is to reach for Twilio or a similar VoIP provider. It makes sense — Twilio is the default for programmable telephony. But VoIP numbers have a fundamental problem for AI agents: they get blocked.
Platforms like Stripe, Coinbase, WhatsApp, Uber, and most financial services use carrier lookup databases to detect VoIP numbers. When a number is flagged as VoIP, the service either rejects it outright or silently fails to deliver the verification SMS. Your agent is stuck at the signup screen with no way forward.
Why Twilio Wasn't Built for Agents
Twilio is excellent infrastructure for human-facing applications — customer notifications, 2FA, contact centers. But it was designed for a world where humans manage the phone numbersand humans read the messages. Here are the specific gaps:
No OTP Auto-Detection
With Twilio, incoming SMS messages are delivered to a webhook as raw text. Your agent has to parse the message body, identify the OTP pattern, and extract the code. This is fragile — every service formats their SMS differently. AgentCall's sms.waitForOTP() handles this automatically with built-in pattern matching across hundreds of known OTP formats.
No Agent Isolation Model
Twilio numbers are tied to an account, not to an agent. If you're running multiple agents, you have to manually manage which agent owns which number, route incoming messages yourself, and prevent cross-talk. There's no concept of agent-scoped resources.
Dashboard-First Workflow
Provisioning a Twilio number still involves the dashboard for many configurations — setting up messaging services, configuring webhooks per number, managing compliance bundles. For agents that need to provision numbers programmatically and on-demand, this friction adds up.
VoIP-Only Number Pool
Twilio's number inventory consists of VoIP numbers. These are fine for sending notifications to humans, but they fail verification checks on the services your agents need to sign up for.
The Telnyx Alternative — And Its Limits
Telnyx offers SIP trunking and a more developer-friendly API, but shares many of Twilio's limitations for the agent use case. Their numbers are still primarily VoIP. They don't offer OTP auto-detection, and there's no agent-native SDK or isolation model.
Both Twilio and Telnyx are great for what they were built for — programmable communications for human-facing products. But AI agents have different requirements.
What Agents Actually Need
After working with dozens of agent developers, we identified the core requirements:
- Real SIM numbers that pass non-VoIP verification checks
- Programmatic provisioning — no dashboard, no compliance forms, just an API call
- Built-in OTP extraction — receive the code, not the raw SMS
- Agent-scoped isolation — each number belongs to one agent
- Event-driven architecture — webhooks for every SMS, call, and OTP event
- Usage-based pricing — no per-seat fees that scale with agent count
Side-by-Side Comparison
| Feature | Twilio | Telnyx | AgentCall |
|---|---|---|---|
| Real SIM numbers | No | No | Yes |
| OTP auto-detection | No | No | Yes |
| Agent isolation | No | No | Yes |
| Programmatic provisioning | Partial | Partial | Full |
| Voice calls | Yes | Yes | Yes |
| SMS inbox per number | No | No | Yes |
| Usage-based pricing | Yes | Yes | Yes |
Making the Switch
If you're currently using Twilio or Telnyx for agent workflows and running into VoIP rejections, the migration is straightforward. AgentCall's SDK mirrors familiar patterns:
// Twilio approach: manual OTP parsing
// const message = await getLatestSMS(twilioNumber);
// const otp = message.body.match(/\d{6}/)?.[0];
// AgentCall: one line
const otp = await client.sms.waitForOTP(numberId);The API handles provisioning, message routing, and OTP extraction — so you can focus on what your agent actually does, not on phone infrastructure.
FAQ
Are Twilio numbers always blocked?
Not always — some services accept VoIP numbers for non-critical verification. But high-security platforms (financial services, identity verification, ride-sharing) consistently reject them. If your agent needs to work everywhere, real SIM numbers are the reliable choice.
Is AgentCall more expensive than Twilio?
For agent use cases, AgentCall is typically cheaper because you're not paying for features you don't use (IVR, call center tools, compliance bundles). Pricing is purely usage-based: you pay for numbers, messages, and minutes.