I'm fully prepared to get flamed for this BUT, I think u are all being massively unreasonable.
I actually find it massively rude and childish to a) think that anything u have to say is anywhere near more important than what the other party has to say or do and b) to deliberately try and trip them up or humiliate them.
When u start a conversation up with some1 who is on their phone, reading a book, watching the tv etc it is actually YOU that is interrupting them. Like a little child, you come in and expect every1 to stop what they are doing and focus on you, just because YOU have something to say? RUUUUDDDEEEE!!!!
I will wander into another room while having a conversation and say "carry on, I'm listening" because yes I AM listening and yes I CAN hear you. point proven by some1 saying "forget it u obviously aren't listening" being fully heard and comprehended from the other room. I am moving around doing other stuff while listening to you because you have chosen a really stupid time to start a conversation and will almost certainly turn round and say, "oh u need to hurry up, ur going to be late" or "are we going or what" if i do stop to have this 'oh so important (to you)' conversation.
I will also actually be listening to someone while having my face in my phone, I can read and type at the same time I am not simple or a child with focus problems. If u speak to me while I am on my phone (or even reading a book), then I am listening but u may not know that because if u are being sarcastic ie "when I have ur attention", then I WILL make out like im not listening and ignore u just to piss u off. If u are having a genuine question or say something that requires a response, then despite having my eyes elsewhere I will respond.
Also, it is incredibly rude and "I'm more important than u" to waste some1s time waffling on. Eg my dad comes shopping with me every week. I will ask "where do u need to go", he will respond everytime with "farm foods cos I need abcdefg, Iceland for hijklmnop..." at which point I will interrupt and say "right let's go then". Because as rude as interuppting might be to some people waffling is even more rude and actually shows that YOU were not listening in the first place because I didn't ask you to give me it shopping list, I asked WHERE u wanted to complete the shopping list.
Staring? also completely rude.
Oh and I am typing this as I'm having a full blown (2 way) conversation with dh about something that happened today and watching my dd trying and failing to play the sly ninja and sneak more felt tips into her bedroom.
The only thing I completely agree with is those who say "Why is this the first I'm hearing about it", or "you never told me", Yes I actually fucking did but as usual u just sat there looking at me (as if u were listening) but had obviously checked out!