Inworld has acquired voice-agent platform Ultravox and already moved its built-in voices onto Realtime TTS-2.

Ultravox is a full voice agent platform. It understands what you said and what you meant, works through it, calls tools, finishes the task, and handles the turn taking and interruptions in real time.

Realtime TTS-2, which now sits behind the built-in Inworld voices on Ultravox, takes direction in plain language.
i.e. with inline instructions you can change pace, tone and emotional delivery for a specific stretch of speech.

So instead of setting a rate to 0.85 for the whole call, the agent can ask the voice to slow down while it walks someone through a complicated step, then go back to normal.