1(Of a phone line) busy — indicates the line is currently in use with another call and can't be reached. Common in automated phone messages.
ごめん、その番号は話し中だったから後でかけ直すね。
Sorry, that number was busy, so I'll call back later.
「ただいま電話が話し中です。しばらくたっておかけ直しください」という案内が流れた。
An automated message played saying, "The line is currently busy. Please try again in a while."
お店の予約ダイヤルはずっと話し中だった。
The restaurant's reservation line was busy the whole time.
2Under discussion or in talks — indicating that a meeting, negotiation, or conversation is currently taking place.
その案件は現在社内で話し中なので、詳しいことはまだ言えない。
That matter is currently under discussion within the company, so I can't give details yet.
両国は条約の内容について話し中だと報じられた。
It was reported that the two countries are in talks about the terms of the treaty.