This is unacceptable behavior in an adult. There must be consequences for bad behavior otherwise we are part of the problem. These people have to understand that things will not simply return to normal as if nothing happened when they decide to communicate properly again. I would write them a simple note stating you are waiting for them and the first thing you will discuss is how they are going to stop this rude and childish behavior. My ex gf used to tell me she did this because it kept her from saying things she'd regret (in response to some imagined slight). From the get go I did not reward her for this by begging for forgiveness for something I didn't say or do. She did not stop, and guess what? She did not get a ring from me.