I can't leave a note.
Why can't you leave a note?
I want to leave a note but I just don't have the time. I'll have to call when I get a chance.
It's nice to leave a note but if you don't have the time to then it's alright. Just make sure you try to get in touch as soon as you can with whatever information you needed to write down.
I'm a busy man. Everyone knows that.
If you do not have time to leave a note, you should find another way to get in touch with them and let them know what it is going on. I get that time is of the essence but it is important that you at least let someone know what is going on.