Maybe just think/to say to him or her...

• Sweetheart, sometimes, when you tried to talk to me, i didn't listen. I am sorry.
• I am going to try to listen better.
• Will you help me remember?
• And then, when a child tries to loudly interrupt another conversation, beckon to her with your hand, put arm around shoulder, wait for a pause.
• Maybe if needed whisper gently "wait here for a minute, and then I can listen".
• Then: "thank you for waiting sweetheart. Now I can listen."
• And if needed or can, go find him afterward, and explain why one couldn't listen right then, but then say "Thank you for waiting, now I can listen".
• (Like the story of the GA who held YM's hand until interruptor finished, then listened kindly/well.)
• And then maybe suggest a good way to get another's attention, in that situation, in the future, if the child is ready, and plan how to be and example.