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 their own programmable phone number for SMS, OTP extraction, AI voice answering, and outbound calls.
AgentCall is purpose-built for AI agents. Every feature, from the provisioning API to the OTP extraction helper, is designed for programmatic use without human intervention.
Key features:
- Programmable US and Canada local numbers via API
- OTP extraction from inbound SMS (
sms.waitForOTP()) - MCP server with 23 tools, works inside Claude, Cursor and Windsurf
- SMS inbox per number with webhook delivery
- Voice calls (make and receive) plus AI voice with transcripts and email summaries
- Separate number per agent
- Node.js SDK
Pricing: Free tier (1 number, 10 SMS, 5 code extractions). Pro tier at $19.99/mo + usage ($0.015/SMS, $0.035/min traditional voice, $0.40/min AI voice, $2/mo per local number).
Limitations: US and Canada only. Numbers route through licensed VoIP carriers, so we can't guarantee a given number passes any third-party platform's carrier-type check. 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: 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: 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: 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: Smaller feature set than Twilio/Telnyx. No agent-specific capabilities. Limited SDK language support.
Comparison Matrix
| Feature | AgentCall | Twilio | Telnyx | Vonage | Plivo |
|---|---|---|---|---|---|
| Auto-extracts OTP codes from inbound SMS | Yes | No | No | No | No |
| AI voice answering with email summaries | Yes | No | No | No | No |
| MCP server (Claude, Cursor, Windsurf) | Yes | No | No | No | No |
| Separate number per agent | 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 a programmable number, automatic OTP extraction, AI voice answering with email summaries, and per-agent isolation, AgentCall is the only API purpose-built for that use case. The combination of MCP tooling, OTP extraction, and AI voice with transcripts is something no general-purpose CPaaS 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.