Maybe you can figure out a way for him to know when it's an emergency and he can't ignore you. For example say that you'll always contact him through text or Whatsapp for general communication stuff, and that you'll only call when there is a serious emergency? That way he knows that when his phone is ringing and it's you that he really needs to pick up because something serious is going on and that it's not something he can read later. Although in all honesty he should understand that this causes you distress if he knows what you are dealing with and he should just answer in general.