It's so nice to hear your voice.

这两个句子都是"主系表"句型.都是用it 作形式主语,不定式作真正的主语.
It's necessary for you to lock the car when you do not use it.中it 作形式主语,is 作连系动词,necessary作表语,
for you to lock the car 作真主,
when you do not use it是时间状语从句.
for you 是不定式的逻辑主语,表示不定式动作的发出者.