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, A2P registration, and how well they support autonomous agent workflows.
This comparison covers general-purpose CPaaS providers. If you are comparing voice AI platforms instead, see 5 Best Phone APIs for AI Agents, which puts AgentCall next to Vapi, Bland.ai, Retell, and Twilio.
Competitor pricing below was verified against each provider's public pricing page on August 9, 2026. Rates change, and every provider on this list adds carrier surcharges on top of the headline number, so confirm current pricing before you commit.
1. AgentCall
Best for: AI agents that need their own programmable phone number for SMS, OTP extraction, AI voice answering, outbound calls, and memory that survives between 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. The other four providers on this list are CPaaS platforms that predate AI agents and expect a human developer to assemble the pieces.
Key features:
- Programmable US and Canada numbers (local, mobile, toll-free) via API
- OTP extraction from inbound SMS (
sms.waitForOTP()) - Hosted MCP server with 60 tools, works inside Claude, Cursor, and Windsurf with no local install
- SMS inbox per number with webhook delivery and readable delivery status
- Two-way AI SMS: an AI answers inbound texts on your behalf and can call your own pricing, booking, or CRM systems mid-conversation through the Action Bridge
- Relay mode: point the number at your own agent and AgentCall runs no LLM of its own, just signs each inbound text to your webhook and delivers your agent's reply whenever it is ready
- Voice calls plus AI voice in 31 languages, with transcripts, post-call email summaries, and transfer to a human
- Auditable Call Memory: returning callers are recognized, and every memory entry quotes the transcript span that produced it
- A2P 10DLC registration handled for you, usually about two hours
- Separate number per agent, so a ban or compromise never spreads
- Call recording at $0.01/min and Node.js SDK plus REST API
Pricing: Free tier with 1 number and no card, 20 outbound SMS/month, 5 voice minutes, 5 OTP extractions/month, and a 5-minute monthly trial of inbound AI voice. Pro is $19.99/mo plus usage: $0.015/SMS, $0.035/min traditional voice, $0.40/min managed AI voice or $0.10/min BYOK with your own OpenAI key, $0.59/min Premium Voice, $0.01/min recording, and $2/mo per local or mobile number ($2.50 toll-free). Cross-call memory is included on every plan. See the pricing section.
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. Higher per-message cost than a pure CPaaS, which matters at high notification volume. No IVR trees, queues, or contact center features: the AI can transfer a live call to a human, but there is no routing or hold-queue layer.
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. We covered one concrete example of that gap in why you don't need an inbound webhook to answer calls.
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: $0.0083 per US SMS segment, inbound and outbound, plus carrier fees of roughly $0.0025 to $0.007 per message depending on the destination carrier. Numbers carry a monthly fee, and A2P 10DLC brand and campaign registration has its own fees.
Limitations: No OTP auto-detection for inbound SMS. No agent isolation concept. No cross-call memory. You own your 10DLC registration and the wait. 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: About $0.004 per message part inbound and outbound, plus carrier fees of roughly $0.0035 to $0.0045 per part. Numbers start around $1.00. Generally cheaper than Twilio for equivalent usage.
Limitations: No OTP auto-detection. No agent-specific features. No cross-call memory. You own your 10DLC registration. 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
- No-code tooling for simple chatbot flows
- Enterprise SLAs
Pricing: Varies significantly by country and is quoted per destination rather than as a single headline rate, so check their current pricing page for the markets you send to. Number rental is charged monthly on top of per-message cost.
Limitations: Complex pricing structure that is hard to forecast. API ergonomics are weaker than Twilio. No OTP auto-detection, no cross-call memory, and 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: About $0.0077 per US SMS on long codes, plus carrier surcharges of roughly $0.0025 to $0.0050 per message. Long code numbers are $0.50/month and toll-free numbers $1.00/month, the cheapest number rental on this list.
Limitations: Smaller feature set than Twilio and Telnyx. No OTP auto-detection, no agent-specific capabilities, no cross-call memory. Limited SDK language support.
Comparison Matrix
| Feature | AgentCall | Twilio | Telnyx | Vonage | Plivo |
|---|---|---|---|---|---|
| A2P 10DLC registration handled for you | Yes, about 2 hours | No, you register with TCR | No, you register with TCR | No, you register with TCR | No, you register with TCR |
| Auto-extracts OTP codes from inbound SMS | Yes | No | No | No | No |
| AI answers inbound texts on your behalf | Yes, with tool calls to your systems | No | No | No | No |
| Relay inbound texts to your own agent | Yes | No | No | No | No |
| AI voice answering with email summaries | Yes | No | No | No | No |
| Cross-call memory with source-backed evidence | Yes | No | No | No | No |
| MCP server (Claude, Cursor, Windsurf) | Yes, 60 tools hosted | No | No | No | No |
| Separate number per agent | Yes, first-class | DIY | DIY | DIY | DIY |
| Delivery status readable per message | Yes, plus error codes | Via status callback | Via status callback | Via status callback | Via status callback |
| Per-message US SMS cost | $0.015 all-in | $0.0083 + carrier fees | ~$0.004 + carrier fees | Varies by destination | ~$0.0077 + carrier fees |
| Number rental | $2.00/mo local | Monthly fee per number | From ~$1.00 | Monthly fee per number | $0.50/mo long code |
| Global reach | US and Canada | 180+ countries | Global | Global, strongest international | Global |
| Call center / IVR | Transfer to human only | Yes | Yes | Yes | Yes |
| Video | No | Yes | No | Yes | No |
| Free tier | Yes, no card | Trial only | Trial only | Trial only | Trial only |
The Registration Gap Nobody Prices In
Every per-message comparison above misses the cost that actually delays launches. Sending application-to-person text to US numbers requires 10DLC brand and campaign registration with The Campaign Registry. On Twilio, Telnyx, Vonage, and Plivo, that registration is yours: you submit the brand, submit the campaign, pay the fees, and wait for approval, commonly days to weeks, before your first message delivers to a real handset. Until then your code is done and your product is not shipped.
AgentCall registers your number on its own campaign instead, which usually completes in about two hours, and every number reports a messaging.state of registered, pending, or action needed so you can watch it rather than guess. The tradeoff is that a new account starts restricted: it can text numbers it owns, anyone who contacted it first, individually verified numbers, and a starter allowance of new recipients. A three-minute business verification lifts the restriction in minutes.
If you are sending millions of notification messages a month, the per-message rate dominates and a CPaaS wins on cost. If you are shipping an agent that needs to send its first text today, the registration path is the whole game.
The Bottom Line
If you're building a contact center, IVR, or human-facing notification system at volume, Twilio remains the safe choice. Telnyx and Plivo are solid alternatives if you want similar features at a lower per-message price, and Vonage is worth a look if your traffic is heavily international.
If you're building AI agents that need a programmable number, automatic OTP extraction, an AI that answers texts and calls with real data from your systems, memory that survives between calls, and A2P registration handled for you, AgentCall is the only API purpose-built for that use case. The combination of MCP tooling, OTP extraction, two-way AI SMS, and auditable call memory is something no general-purpose CPaaS offers today.
For more background, see Why AI Agents Need Phone Numbers and How to Give Your AI Agent a Phone Number That Remembers Every Call.
FAQ
Which API has the best documentation?
Twilio has the most comprehensive docs by volume, and its community answers are the reason many teams stay. AgentCall has the most focused docs for the agent use case: fewer pages, but every feature has a working code example, and the hosted MCP server means an AI coding assistant can read the tools directly instead of reading docs at all.
Can I use multiple providers together?
Yes. A common split is Twilio or Telnyx for high-volume human-facing notifications and AgentCall for agent workflows that need OTP extraction, AI voice, or cross-call memory. The APIs are independent and coexist in the same application.
How long until I can send my first A2P text?
On Twilio, Telnyx, Vonage, and Plivo you register your own brand and campaign with The Campaign Registry and wait for approval, commonly days to weeks, before an application-to-person message to a US number will deliver. AgentCall registers your number on its own campaign instead, which usually completes in about two hours. New AgentCall accounts start restricted to numbers they own, anyone who texted them first, individually verified numbers, and a starter allowance of new recipients, and a three-minute business verification lifts that in minutes.
Which phone API is cheapest per SMS?
On raw per-message rates, Telnyx is lowest at about $0.004 per message part, followed by Plivo at about $0.0077 and Twilio at $0.0083, all before carrier surcharges that add roughly $0.0025 to $0.007 per message. AgentCall is $0.015 per message with no separate carrier pass-through, which is higher per unit but includes A2P registration, delivery status, OTP extraction, and the AI reply layer. For low-volume agent workloads the per-message rate rarely dominates the bill; for high-volume notification sending it does.
Do these APIs tell me whether a text was actually delivered?
All of them expose delivery status in some form, usually through a status callback webhook you have to build and host. AgentCall also exposes it as a simple read: GET /v1/sms/:messageId returns a status that moves from queued to sent to delivered or failed, with an error code on failure. This matters more for agents than for people, because an agent sends unattended and nothing notices silence unless it checks.