Features, pricing, and fit
Compare ConversationRelay and Line on voice capabilities, pricing, and fit.
Compare Twilio ConversationRelay and Cartesia Line for AI phone agents, Voice AI platforms, then match channels, workflows, and deployment models to your call flow.
Programmable Voice component that connects a TwiML ConversationRelay session to an application WebSocket for STT/TTS, language settings, DTMF, and call events.
Developers building their own voice-agent logic while keeping telephony, media handling, and call control in Twilio.
Explore its buying criteria: AI phone agents · Voice AI platforms.
Managed Line runtime for voice agents with no-code Agent Builder, SDK-defined reasoning, Cartesia Ink STT, Sonic TTS, deployment, and observability.
Teams that want managed audio plumbing while retaining code-level control over reasoning, tools, and external APIs.
Explore its buying criteria: AI phone agents · Voice AI platforms.
Both profiles checked 2026-09-04.
Use the table below to compare each provider’s documented scope.
Before you decide
Twilio ConversationRelay: Usage-based Programmable Voice minutes and phone numbers, plus ConversationRelay per-minute charges; model, provider, and other call features are separate. Cartesia Line: Not publicly documented Ask both vendors to quote the same expected workload, required channels, and implementation scope. Include required subscriptions and usage charges before comparing the total; the listed starting prices alone may cover different work.
The recorded knowledge and data sources for Twilio ConversationRelay include Not publicly documented. For Cartesia Line, they include Not publicly documented. Ask each provider to demonstrate access for the exact systems you use, with permissions, freshness, and failure behavior. A source missing from this directory is not proof that a provider cannot support it.
Use the same representative calls, approved instructions, and escalation rules in both pilots. How does it handle interruption and uncertainty? Which actions can it complete, and which require approval? What context reaches a human during transfer? Record conversation quality, completed tasks, human handoffs, and total cost in the buyer worksheet. Check these specific cautions before launch: Twilio ConversationRelay: Not a packaged agent. Region, carrier, recording, compliance, model, and application responsibilities remain with the implementer. Cartesia Line: Confirm product availability, carrier coverage, models, data handling, and generally available features.
Record your requirements and pilot results in the buyer worksheet →
Swipe or scroll the table to see every column.
| Criterion | ConversationRelay | Line |
|---|---|---|
| Provider type | Telephony voice-agent runtime | Voice agent platform and speech API |
| Best fit | Developers building their own voice-agent logic while keeping telephony, media handling, and call control in Twilio. | Teams that want managed audio plumbing while retaining code-level control over reasoning, tools, and external APIs. |
| Key capabilities | phone audio streaming; speech interface; interruptions; WebSocket protocol; TwiML controls | realtime voices; agent configuration; telephony; tools; API |
| Team sizes | startup; mid-market; enterprise; developer | startup; mid-market; enterprise; developer |
| Call surfaces | phone / PSTN; WebSocket | phone / PSTN; API / SDK |
| Voice stack | Cascaded speech interface with developer-supplied LLM | Cascaded STT-LLM-TTS using Cartesia TTS |
| Telephony | Twilio Programmable Voice; phone numbers; SIP | Not publicly documented |
| Call controls | Not publicly documented | Not publicly documented |
| Languages | Not publicly documented | Not publicly documented |
| Integrations | Not publicly documented | Not publicly documented |
| Knowledge and data | Not publicly documented | Not publicly documented |
| Pricing model | Usage-based Programmable Voice minutes and phone numbers, plus ConversationRelay per-minute charges; model, provider, and other call features are separate. | Not publicly documented |
| Pricing signal | ConversationRelay $0.07/min; US local outbound $0.014, inbound $0.0085, SIP $0.004, number $1.15/month. | Approximately $0.06/min plus about $0.014/min managed telephony; verify current calculator. |
| Free plan or trial | Not publicly documented | Not publicly documented |
| What stands out | ConversationRelay handles the live call’s STT/TTS and session transport, then sends prompts, audio-control events, and optional DTMF over a required secure WebSocket to the application. | Line separates reasoning from audio operations: it offers a no-code builder or SDK for custom logic while managing deployment/observability and pairing Ink with Sonic. |
| Considerations | Not a packaged agent. Region, carrier, recording, compliance, model, and application responsibilities remain with the implementer. | Confirm product availability, carrier coverage, models, data handling, and generally available features. |
Developers building their own voice-agent logic while keeping telephony, media handling, and call control in Twilio.
ConversationRelay handles the live call’s STT/TTS and session transport, then sends prompts, audio-control events, and optional DTMF over a required secure WebSocket to the application.
Teams that want managed audio plumbing while retaining code-level control over reasoning, tools, and external APIs.
Line separates reasoning from audio operations: it offers a no-code builder or SDK for custom logic while managing deployment/observability and pairing Ink with Sonic.
Verify current pricing, limits, security, and required integrations directly with each provider before purchasing.
Test recognition, speech quality, interruptions, language switching, tool use, and human transfer with your own calls in the required language. Availability can vary by model, plan, and phone route.
DocsBot documents multilingual voice workflows and Japanese support. Verify the exact voice, phone path, and transfer behavior in your pilot. Review DocsBot language information ↗
Other providers’ language claims reflect cited vendor material; confirm Japanese and other required languages directly with each provider.
Listing does not imply endorsement or a performance guarantee. This directory is published by DocsBot. Editorial policy
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