Choosing a phone number API depends on what you're building. If you're building for AI agents, you need different things than if you're building a call center. This guide compares the five most popular options across features, pricing, and how well they support autonomous agent workflows.
1. AgentCall
Best for: AI agents that need real phone numbers for verification, SMS, and voice.
AgentCall is purpose-built for AI agents. Every feature — from the provisioning API to OTP auto-detection — is designed for programmatic use without human intervention. Numbers are real SIM-based, meaning they pass the carrier verification checks that VoIP numbers fail.
Key features:
- Real SIM number provisioning via API
- Built-in OTP extraction (
sms.waitForOTP()) - SMS inbox per number with webhook delivery
- Voice calls (make and receive)
- Agent-scoped number isolation
- Node.js and Python SDKs
Pricing: Free tier (1 number, 50 SMS). Pro tier at $19.99/mo + usage ($0.015/SMS, $0.035/min, $2-8/mo per number).
Limitations: Newer platform with a smaller country coverage compared to Twilio. No IVR or call center features (by design).
2. Twilio
Best for: Human-facing products that need notifications, 2FA, and contact center features.
Twilio is the industry standard for programmable communications. It offers the broadest feature set: SMS, voice, video, email (via SendGrid), WhatsApp, and more. The tradeoff is complexity — Twilio's API surface is massive, and configuring it for simple agent use cases involves more setup than necessary.
Key features:
- Massive global number inventory
- SMS, MMS, voice, video, WhatsApp
- Programmable IVR and call routing
- Verify API for human 2FA
- Extensive documentation and community
Pricing: ~$1.15/month per number + $0.0079/SMS segment. Verify API is $0.05/verification.
Limitations: VoIP-only numbers get blocked by many services. No OTP auto-detection for inbound SMS. No agent isolation concept. Dashboard-heavy configuration.
3. Telnyx
Best for: Developers who want a Twilio alternative with better pricing and SIP trunking.
Telnyx positions itself as the developer-friendly alternative to Twilio, with a focus on infrastructure control and competitive pricing. Their Mission Control portal gives more visibility into carrier-level operations than most competitors.
Key features:
- SMS and voice APIs
- SIP trunking and elastic SIP
- Global number provisioning
- Competitive per-message pricing
- Programmable fax (niche but unique)
Pricing: ~$1.00/month per number + $0.004/SMS. Generally 30-50% cheaper than Twilio for equivalent usage.
Limitations: Still VoIP numbers. No OTP auto-detection. No agent-specific features. Smaller community than Twilio.
4. Vonage (formerly Nexmo)
Best for: Enterprise teams that need a full communications platform with strong international coverage.
Vonage offers a broad suite of communication APIs under the Vonage API brand. Their strength is international coverage and enterprise-grade reliability. The developer experience has improved but still lags behind Twilio and Telnyx.
Key features:
- SMS, voice, video, and messaging APIs
- Strong international number availability
- Verify API for human 2FA
- AI Studio for no-code chatbots
- Enterprise SLAs
Pricing: Varies by country. US SMS starts at ~$0.0068/message. Number rental from ~$1.00/month.
Limitations: VoIP numbers. Complex pricing structure. API ergonomics are weaker than Twilio. Not designed for autonomous agent workflows.
5. Plivo
Best for: Cost-sensitive teams that need basic SMS and voice without enterprise overhead.
Plivo is a no-frills SMS and voice API. It's straightforward, affordable, and reliable for basic use cases. If you need to send notifications or build a simple IVR, Plivo gets the job done without the complexity of Twilio.
Key features:
- SMS and voice APIs
- Simple REST API design
- Competitive pricing
- PHLO visual workflow builder
- Good deliverability for notifications
Pricing: ~$0.80/month per number + $0.005/SMS. One of the most affordable options.
Limitations: VoIP numbers. Smaller feature set than Twilio/Telnyx. No agent-specific capabilities. Limited SDK language support.
Comparison Matrix
| Feature | AgentCall | Twilio | Telnyx | Vonage | Plivo |
|---|---|---|---|---|---|
| Real SIM numbers | Yes | No | No | No | No |
| OTP auto-detect | Yes | No | No | No | No |
| Agent isolation | Yes | No | No | No | No |
| SMS & Voice | Yes | Yes | Yes | Yes | Yes |
| Call center / IVR | No | Yes | Yes | Yes | Yes |
| Video | No | Yes | No | Yes | No |
| Free tier | Yes | Trial only | Trial only | Trial only | Trial only |
The Bottom Line
If you're building a contact center, IVR, or human-facing notification system, Twilio remains the safe choice. Telnyx and Plivo are solid alternatives if you want similar features at a lower price.
If you're building AI agents that need to autonomously sign up for services, verify phone numbers, send SMS, and make calls — AgentCall is the only API purpose-built for that use case. Real SIM numbers, automatic OTP extraction, and agent-native isolation are features no other provider offers today.
FAQ
Which API has the best documentation?
Twilio has the most comprehensive docs by volume. AgentCall has the most focused docs for the agent use case — fewer pages, but everything you need is covered with working code examples.
Can I use multiple providers together?
Yes. Some teams use Twilio for customer-facing notifications and AgentCall for agent workflows. The APIs are independent and can coexist in the same application.