Getting a phone call always feels so rude. It's like it's saying, "Whatever you are doing, stop RIGHT NOW and listen to me. NOW! NOW! I'm giving you TEN SECONDS! LISTEN TO ME."
Just don’t answer, or, better yet, stick your phone on silent or do not disturb when you are busy. you’re projecting that emotion on to the caller, I see an incoming call as an indication that someone would like to speak to me, it a demand for attention. If I can’t take the call I won’t answer and will call them back (or email to arrange a more suitable time) when I can.
> email to arrange a more suitable time
Or better yet, provide the important details over email and then if there is some unimaginable reason why an audio medium is still required to convey information (identify a bird call? place a long distance call with a whistle?) then contact over the phone.