Research

👁️‍🗨️ Voice Intelligence

The leading voice agent APIs offer lifelike speech synthesis, robust conversational logic, and scalable telephony integration. Choose based on your specific needs:

Top Recommendations

1. Lindy

What does it do? Lindy is a no-code voice agent platform that can take calls, hold real conversations, qualify leads, send follow-ups, and update your systems without human input.

Who is it for? Perfect for teams that deal with sales calls, support tickets, recruiting, or client onboarding.

Lindy can make and take real phone calls. And yes, it ACTUALLY sounds like a person. Built with drag-and-drop flows, no coding required.

Key Features:

  • Built-in call summaries, follow-ups, and Slack alerts
  • Handles real phone calls with natural conversation flow
  • Can search internal docs and update databases mid-call
  • Multiple simultaneous calls capability
  • Automatic conversation logging and database updates

Pros:

  • No-code platform with visual flow builder
  • Real phone call capabilities with natural conversation
  • Built-in integrations (Slack, databases, knowledge base)
  • Handles multiple concurrent calls

Cons:

  • Call features not included in free plan
  • Needs paid phone number for call features

Pricing:

  • Free plan: 400 tasks/month, 1M character knowledge base
  • Pro ($49.99/month): 5,000 tasks/month, access to call features, 20M character knowledge base
  • Business ($299.99/month): 30,000 tasks/month, premium phone call automation, priority support

2. Vapi - Best for omnichannel support

What does it do? Vapi is a developer-focused voice AI platform that creates highly customizable voice agents.

Who is it for? Particularly suited for businesses that need deep customization, integration with existing systems, and want to handle high volumes of concurrent calls.

Key Features:

  • API-first approach with deep flexibility
  • Real-time call handling with impressively low latency
  • Use your own models for speech, transcription, and LLMs
  • Scales easily to over a million concurrent calls
  • Route calls, handle interruptions mid-sentence, pass context to external APIs

Pros:

  • Made for developers with great flexibility and full control over logic
  • Real-time call handling with impressively low latency
  • API-first setup that fits cleanly into modern stacks
  • Scales easily to over a million concurrent calls

Cons:

  • You'll need to handle your own frontend and call logic
  • Not beginner-friendly, requires coding and API knowledge
  • Costs can add up quickly for high-volume use cases

Pricing:

  • Free trial: $10 in free credits when you sign up
  • Platform fee: $0.05/minute (billed per second)
  • Phone numbers: $2/month
  • Additional costs: Based on usage of third-party models (e.g. OpenAI, ElevenLabs)

Key Providers

ElevenLabs - Best for expressive AI voices What does it do? ElevenLabs is a voice generation platform that specializes in producing incredibly lifelike, emotionally rich speech.

Who is it for? Perfect for teams who are already building AI voice agents and want them to sound genuinely human.

  • Exceptionally lifelike, emotional speech synthesis
  • Voice cloning and multi-language support
  • API-first approach (requires pairing with agent logic)
  • Free plan available, paid from $5/mo

PlayHT - Best for lifelike conversations

  • Real-time, human-like speech with strong NLP
  • Multi-language support and easy business integration

OpenAI Voice Agent SDK - Best for end-to-end automation

  • Complete agent SDK with whisper-1 (STT) and tts-1 (TTS)
  • Context management and workflow handoffs

Retell AI - Best for full voice agent platform

  • Build, test, deploy, and monitor production agents
  • SIP integration with pay-as-you-go pricing ($0.07/min)

Plivo - Best for customizable business agents

  • Choose your LLM/TTS with real-time response (30ms)
  • 99.99% uptime, integrates with OpenAI/ElevenLabs ($0.003/min per stream)

Deepgram - Best for speech-to-text & analysis

  • Fast, accurate STT with real-time transcription
  • Multi-language support and flexible API

Traditional Telephony: Twilio, Vonage, MessageBird

  • Global coverage with call control, TTS, STT, analytics
  • Strong developer ecosystem for programmable voice

Quick Recommendations

  • Overall best: Lindy (no-code, full-featured)
  • Developer-focused: Vapi (highly customizable, API-first)
  • Voice quality: ElevenLabs or PlayHT
  • End-to-end platform: OpenAI SDK, Retell AI, or Plivo
  • Speech recognition: Deepgram
  • Telephony infrastructure: Twilio, Vonage, or Plivo

All platforms offer robust APIs and documentation for deploying advanced voice agents in 2025.

ProviderBest ForKey FeaturesPricing/Notes
ElevenLabsExpressive, emotional voice synthesisExceptionally lifelike, emotional voices; supports many languages; voice cloning; API-firstFree plan; paid from $5/mo; requires pairing with agent logic tools
PlayHTLifelike conversationsReal-time, human-like speech; strong NLP; multi-language; easy integration for business useSee provider for pricing
OpenAI Voice Agent SDKConversational logic & workflow automationEnd-to-end agent SDK; "whisper-1" for STT, "tts-1" for TTS; context management; easy handoffsSee provider for pricing
Retell AIFull voice agent platformBuild, test, deploy, and monitor production-ready agents; SIP integration; pay-as-you-go pricing$0.07/min, no platform fees
PlivoCustomizable business voice agentsChoose LLM/TTS; real-time response (30ms); 99.99% uptime; easy integration (OpenAI, ElevenLabs)$0.003/min per stream
DeepgramSpeech-to-text & voice analysisFast, accurate STT; supports many languages; real-time transcription; flexible APISee provider for pricing
AgentStationCustom agent logic with LLMsBuild agents using GPT-4, integrate with Twilio/SignalWire; high customizationSee provider for pricing
InfobipVoice, IVR, and call routingVoice calls, IVR, call recording, multi-channel messaging; high customizationFrom $0.002/min
Twilio, Vonage, MessageBirdTelephony & programmable voiceGlobal coverage; call control, TTS, STT, analytics; strong developer ecosystemSee provider for pricing