Final answer:
Mildred is relieved when the phone rings during Montag's passionate speech in 'Fahrenheit 451' because it allows her to escape an uncomfortable conversation about society's deep issues.
Step-by-step explanation:
In Fahrenheit 451, Mildred is grateful when the phone rings during Montag's tirade about the truth because it gives her an opportunity to avoid the uncomfortable conversation. Mildred is depicted throughout the novel as a character who prefers the shallow interactions of her 'family' on the screens to confronting the reality of her life and the societal issues around her. Her relief at the phone's interruption signifies her desire to escape from the meaningful but difficult conversation with her husband, Montag, about books, knowledge, and the oppressive nature of their society.