noun
telephone receiver; mouthpiece
Refers to the part of a telephone you speak into or listen from; often used in phrases like 電話口に出る (come to the phone).
彼は電話口で大きな声で話した。
He spoke loudly into the telephone receiver.
電話口から聞こえる声が小さかった。
The voice coming through the receiver was faint.
noun
making or receiving a call; on the phone
Used in set expressions like 電話口に出る (answer the phone) or 電話口で話す (talk on the phone); emphasizes the act of being on the line.
See also: 電話口に出る (でんわぐちにでる)
母はすぐに電話口に出た。
My mother came to the phone right away.
電話口で長話をするのは好きではない。
I don't like having long conversations on the phone.
電話 is the general word for telephone or phone call; 電話口 is more specific to the receiver or the point of communication.
Compound of 電話 (telephone) and 口 (mouth, opening). The exact historical derivation is uncertain, but the word literally suggests the 'mouth' of the telephone.