不知道對不對...

今後我將為貴司主要聯絡窗口，萬一無法聯絡上我時，請聯絡次要窗口的XXX先生

I will be the window of contact for XXX Company, in case of my absence, please contacts Mr. XXX as he will be my deputy while I am away.

contact 比 window of contact 還常用..