In 1965 in the US, AT&T released a new model phone called the Trimline. At the time, AT&T didn't allow (or at least support) anyone else's phones on their network. One of the options for the Trimline phone was a wall mount, which (I'm pretty sure) was the only wallmount phone available. The mount was accomplished by having a combination mounting plate and phone jack. The phone was hung on the wall by sliding its slots onto two metal studs permanently affixed to the mounting plate, and the jack, located about halfway between the studs, received a plug mounted directly to the back of the phone which could travel half an inch or so up and down to allow for the mounting.

This setup became something of a de facto standard, and virtually every wall-mount phone sold in the US has the mounting points matching the spacing of those original studs from 1965, though most use a short cable to plug in rather than a bound one like from the original Trimline.

So that's what he's talking about.