1.
on a call; busy; in the middle of a phone call
The state of being engaged in a telephone conversation, or a phone line being in use.
今、通話中です。
I'm on a call right now.
A compound of 通話 (telephone call/conversation) and 中 (in the middle of/during). Used both as a status indicator and in conversation to indicate someone is on the phone.
On older phones and landlines, calling a 通話中 number produces a busy signal (話中音). On smartphones, the term appears as a screen indicator during active calls. The ~中 suffix is extremely productive and can attach to many nouns to mean "in the process of."