There are some expressions that you have to use in the telephone conversation:
At the very beginning of the telephone call, you may say “Hello! May I speak to … , please?” In Chinese, it is 你好! 請……聽電話。 (nǐ hɑ̌o!qǐng……tīng diɑ̀n huɑ̀。) .
When someone is there, you may answer “Speaking. Who’s calling?” In Chinese, it is 我是。你是誰? (wǒ shì。nǐ shì shéi?)
If you are looking for the person to answer the call, you can say “Just a moment, please.” In Chinese it is 請等等 (qǐng děng děng。)