The fare rules for the ticket are very important here and would tell you whether there's any chance of salvaging any value (even if that's the ability to later use a segment you won't likely use). If it's a U.S. based carrier and the fare is close to the lowest available, I'd say it's almost a certainty that you'll forfeit any remaining value by missing the flight, and that your only other option is to pay a $100-150 change fee (+fare difference) to move the return to another date that you'd also be likely to miss.

If you're fine with just not flying the segment and having no more money change hands, I'd say just miss the flight and say nothing. The problem with calling a person is that they might decide you're trying to pull something to avoid paying more or are otherwise breaking the rules (or maybe they're just new on the job) and will put you on a long hold while they talk to a supervisor. If you get that, odds are excellent it'll lead to an outcome worse than keeping your mouth shut. It's one of those cases where avoiding human contact is the better course. As they say in the South, don't fix it if it ain't broke.