If you’ve been wondering what could knock out one of the United States’ three big cellular carriers’ ability to deliver calls and text messages — and keep it that way for most of an entire day — T-Mobile now has a partial answer that pertains to its extensive nationwide outage Monday. The short version, if we’re reading this correctly: a fiber-optic circuit failed, and its backup circuit also failed, which caused a chain reaction that strained the network to the point that many calls and texts couldn’t make it through.