My friend does this all the time, she'll call me (on speakerphone) then her dh or teen will interrupt and instead of saying 'I'm on the phone' will enter into a conversation with them. It's never a short answer it's almost as if she's forgotten I'm on the phone and at times she'll tell me to 'hang on a minute'
Of course if it's an emergency do what you have to do but to answer questions about what's for dinner, where such and such is all mundane things that can wait. Is it me or is this just so bloody rude?
I'm going to hang up next time.