留守電を残すのは時間が短いので速く要約しないといけないって英語でなんて言うの？

Question

It's difficult for me to leave a voice message, because it's short time we can leave a message. We have to speak quickly and make a brief message.

Accepted Answer

It's hard to say everything I need to say on your voice mail because they don't give you much time so I need to keep it really brief. 
「（留守電を残せる）時間は短いので、留守電に言いたいことを全部言うのは難しい。簡潔にする必要がある。」
「留守電」は voice mail 
keep brief で「簡潔にする・手短にする」

ご参考になれば幸いです！