통화 중이에요 · 통화 중이라서 (because the line is busy)
on the phone, (the line is) busy
Example
전화했는데 계속 통화 중이었어요.
I called, but the line was busy the whole time.
Word parts
통 go through+화 talk+중 middle
Literally 'in the middle of a call'; -중 = in the middle of: 회의 중, 수업 중.