There is no master switch to hunt for in Mando. Voice is enabled by default on every Customer Service app, and calls are answered from the Business plan and on every free trial. Light and Growth do not include voice. Turning it on is not the step that makes calls work, so here is what actually does.
The Enable voice switch
On the Voice page, the first card holds an Enable voice switch. It is already on. It reads "Voice agent is active for this app." Leave it on. Turning it off is the kill switch, not the setup step, and the cards below Who answers your calls grey out while it is off.
If you did turn it off, turn it back on and save before configuring anything else. Saving is an admin action, and there is no Save button: an edit raises a Save prompt at the bottom of the page.
What actually makes it answer
Set who answers. In Who answers your calls, choose the language and accent, name, greeting and voice. Without this the agent falls back to a default voice and calls itself "AI assistant". See Set up who answers your calls in Mando.
Keep credits above 5. A call needs at least 5 credits to start and uses 5 a minute. See What counts as a message credit in Mando.
Connect a phone number, for phone calls. Web calls need no number. See Connect a Twilio number to your AI agent.
Show the call button, for web calls. The widget entry point is controlled separately. See Show or hide the call button in your widget and Let visitors call your AI from the chat widget.
Test it. Under Test the voice, press the microphone and talk. The call uses your saved settings.
What callers get when voice is off
A web visitor who tries to start a call sees "Voice calls are currently disabled for this app."
A phone caller hears the line connect and then go silent, with no message. Turn voice back on to restore both.