What is the age when children understand not to make noise when I am on the phone?

by Fed Up
(Ithaca, NY)

I am a single mother who works from home. I have a son who is 3 years and 9 months old.

I have repeatedly told him not to make noise (play his instruments, hit the walls, bang his toys) or come and talk to me when I am on the phone.

Today, the phone rang and when I answered it, my son decided it was time to play his kazoo. I smacked him on the cheek and sent him to time out.

My mom, who is here visiting, witnessed this and accused me of child abuse. I told her she isn't here on a daily basis and does not know how many times I have had this conversation with my son over the last 6+ months.

I am at my wit's end with him not listening to me. Is this just the age or is he developmentally disabled that he cannot remember things?

Fed up in Ithaca, NY