In the middle of something: while something is happening; during something
真っ最中、真っ只中、物事が起きている最中、物事の間に

Manager: Hey Boss, may I speak to you for a minute?

Boss: Not now, I'm in the middle of a phone call. I'll talk to you afterwards.

マネージャー：あの、少しだけお話ししても良いですか？
上司：今はだめだな、電話の真っ最中だ。後で話そう