Being in the middle of a phone call; also indicates a line is busy (in use).
ごめん、今電話中だから後でかけ直すね。
Sorry, I'm on the phone right now, so I'll call you back later.
その番号は電話中のアナウンスが流れた。
A busy tone/announcement was played for that number.
担当者が電話中のため、少々お待ちください。
The person in charge is on the phone, please wait a moment.
通話が長引いていたので、彼女はずっと電話中だった。
Her call went on for a long time, so she was on the phone the whole time.