Source: The Great Interpretation of Dreams (Ibn Sirin)
Answering in a dream indicates the acceptance of a prayer or obedience. Answering a call indicates responding to a command.
Source: Perfuming Humanity in the Expression of Dreams (Al-Nabulsi)
Answering indicates the fulfillment of needs and achieving one's goal.
Source: Signs in the Science of Expressions (Ibn Shaheen)
Answering is interpreted as goodness. Whoever sees himself answering will attain goodness.