telephone receiver; telephone mouthpiece
making a telephone call; receiving a telephone call
電話口の向こうから、子供の声が聞こえた。
I heard a child's voice from the other end of the phone.
山田さん、電話口です。
Yamada-san, it's for you (on the phone).
電話口で相手の言うことがよく聞き取れなかった。
I couldn't quite make out what the person on the other end was saying.