At least there's a toll-free number

As a charter subscriber to the London Review of Books, I've been receiving it for decades. It's true that, before I lived at this street address, I lived at another address on the same street, sharing three of four digits and in the same order. No matter how many change-of-address forms were sent, it took about a decade to get all that straightened out. But today there came a form post card reporting entirely in upper-case letters, "The Post Office has notified us that there is a problem delivering mail to the address printed at right." From there it goes on and on. What can be the problem? There's no way to find out immediately, since the toll-free number has a voicemail message reporting that, because it's a holiday (today?), there's nobody to respond to the call. And the name and the address shown on the card have always worked fine. Perhaps a holiday substitute letter carrier delivered the periodical to the wrong address and the occupant made some mark on it? Creepy Amazon provides no toll-free number where anyone is likely to find it and also fails to respond to correspondence in any form, for the most part. The WSJ this week mentioned Austin's own Ellen Hobbs and her posting of this elusive number at Cliche Ideas.