Everyone took their phone with them when they left. no...that doesn't make any sense because there would still be landlines in the office buildings and in some homes/businesses. For a question like this, I find the clearest answer can be found by simply applying Occam's razor. Using this, the simplest reasonable explanation is:
As the zombie apocalypse began taking hold and war broke out, people started using phones less and less. Not over years, but say a few months. The phone companies revenue drop was significant, and they needed a way to maintain quarterly profit expansion. First they tried stock market manipulation, lying about customer numbers, short selling phone stocks, "the usual", etc, but it wasn't profitable enough, so they made a big fanfare of hiring new CEO's to "shake things up" and immediately borrowed and sunk billions into multimedia advertising campaigns to convince the remaining people alive (there were still quite few as things heated up) that the zombie issue was really caused by the new 6G cell networks (even though there was no such thing as 6G), and that the only path to survival (other than food, water, a strong internet connection, etc) was to eschew cell phones. Once everyone's cell phones ended up in the trash and were carted away to landfills, phase 2 of "Operation Phone Home" began, where the phone companies sent all the alive customers (the paying customer base was still shrinking rapidly) an empty box saying that the customer would be paid in cash for every phone put into the box and mailed back to an address (it also ultimately went to a landfill in another country). The ruse was that the phone companies never planned on paying anyone, but figured if everyone had no phones, they could quickly (and reasonably) go bankrupt and pocket the money they had borrowed in the advertising campaigns they never spent, because they borrowed billions and they *spent billions on paper* but in reality they only *really* spent hundreds of millions, which the difference was routed around and pocketed by the suits. They figured by the time anyone left alive may have figured out the scam, society would have collapsed anyway, they would be set on their own zombie free private islands. Maybe some of them got to spend some of that money, or maybe they all died and its still on some dead computer hard drive in the basement of a building. The important thing is that this is where all the phones obviously went.
Anyway, using the same reasonable deductive logic, the answer also becomes clear for all the pencils, 3 ring binders, desk calendars, loose pens, staplers, etc. Every one of those companies (Big Stapler, Big Desk Calendar, Big Loose Pen, etc.) came to the same conclusions, performed the same massive loan -> blame 6G cell networks -> claim bankruptcy -> profit the difference bait and switch. The last few months of the fall of civilization was mostly a bizarre explosion of advertising fanfare with delivery trucks running wild with all the people shipping their stuff off hoping for cash.
Now that's what I call all tied up, neat, and with a bow.
