Final answer:
The sentence using an anecdote is the one sharing a personal story about the author's best friend walking out on them due to excessive phone use.
Step-by-step explanation:
The sentence that uses an anecdote as a rhetorical device is: "Many people think that they have to check their phones and be connected at all times. I was one of these people, until my best friend walked out on me during our last meal together before college; after that, I vowed to be more present." This sentence introduces a brief personal story to illustrate a point about the negative impact of excessive phone use on relationships, making the topic more relatable to the reader.