If it is doing a tool call, it has to convert the speech to text or at least a JSON object of the necessary parameters for the tool and convert the result to speech doesn’t it? Is it truly speech to speech then?

It's all tokens at the end of the day, not really text or video or audio, just like everything on a machine is just bits of 1s and 0s and it's up to the program to interpret them as a certain file format. These models are more speech-to-speech (+ text) in that they can recognize text tokens too. So the flow is, you ask it something, then,

Audio Tokens: "Let me check that for you..." (Sent to the speaker)

Special Token: [CALL_TOOL: get_weather]

Text Tokens: {"location": "Seattle, WA"}

Special Token: [STOP]

The orchestrator of the model catches the CALL_TOOL and then calls the tool, then injects this into the context of the audio model which then generates new tokens based on that.