What immediate action should you take when operating a boat if you hear fog signal from another boat you cannot see?

If you hear a fog signal from another boat you cannot see, you should immediately reduce your speed and maintain a proper lookout. It's crucial to evaluate your surroundings and be prepared to take evasive action if needed. Additionally, use your navigation lights and sound signals to make your presence known to nearby vessels. Finally, be ready to stop or anchor if visibility remains poor.