I don't think that kids grasp easily the idea of interrupting. When they were younger we used the idea of the kids putting their hands on our arm as a signal that they wanted to talk to us. We promised them that we would get to them within a few minutes.
But now they are older, and I think they could wait longer than a few minutes on many things. When I am on the phone, if their Daddy and I are having a talk or if I am helping another sibling with school work and they need me (or Daddy) we can use this system.
I saw a version of this somewhere on a blog, but now can't find it to give credit. I have also used systems like this in the classroom, but never thought of adopting it for the home.
They remove their clip, and come and clip it on my shirt. I explained that I can't forget them, because their clip will remind me. When I have the next free moment, I will come to them and see what they needed. In the meantime, I gave them a list of approved activities to occupy them while they wait.