Siri won't call on speaker

I had an issue couple months ago with incoming calls that won't go to my earbud but stay on the phone. Googling it, I found the solution (change something in the settings) that actually worked, but I don't remember what it was. But since then, when I'm driving (no earbud used), asking Siri to call someone "on speaker" she repeats my request, including "on speaker" (I also tried "on speakerphone") — but the call does not go to the speaker, it stays the phone.

What should I change?

(my iOS is the latest)

Thanks!