● This phone must be broken. It seems that the person on the other end of the line can hear my voice, but I can't hear theirs at all. ● It looks like this phone is broken. You can hear the other person's voice, but they can't hear you at all.
「(電話の)相手」は the person on the other end of the line でいいでしょう。
My car was OK, but the other guy's was totaled in the accident (僕の車は大丈夫だったが、相手の車は事故でめちゃくちゃになっていた)