Enhance Dialogue is in iOS18 for when the user is playing back media (video or podcasts) on their iPhone or iPad.
It does NOT work for other functions in the iPhone, e.g. when I get a phone call on my iPhone from someone in a noisy restaurant.
In an inverse situation, I may be the person in the noisy restaurant and I make a phone call on my iPhone. In that situation, I can opt to turn on Voice Isolation so my voice is clearer to the person I am calling.