He had already left his office when I called him.
One of the meanings is the same as the kanji, previously, but it can also mean already.
The reading is different from what you learned with the kanji. But as I told you already, I'll be there on Su-day (すで). It's like Sunday, but not quite the same. Imagine your confusion when someone says this over and over, but you don't know what Suday is.