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In-call TTS plays synthesized speech during live voice calls. Two integration paths:

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Voice API

Use the speak command to play TTS on an active call: 
curl --location 'https://api.telnyx.com/v2/calls/{call_control_id}/actions/speak' \
--header 'Content-Type: application/json' \
--header 'Authorization: Bearer YOUR_API_KEY' \
--data '{
  "voice": "Telnyx.Ultra.3e1ed423-17e5-4773-b87c-25b031106e41"
}'
See Voice API docs for the full command reference.

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TeXML

Use the <Say> element: 
<Response>
  <Say voice="Telnyx.NaturalHD.astra">Your appointment is confirmed for tomorrow at 3 PM.</Say>
</Response>
See TeXML docs for the full <Say> reference.

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AI Assistants

AI Assistants use TTS for voice output. Configure the voice model in assistant settings. See AI Assistants for voice configuration.

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Voice Selection

In-call TTS uses the same voice format as WebSocket and REST: 
Provider.Model.VoiceId
All models (including Ultra) are available for in-call playback.