A study by Dan Ariely, the Professor of Psychology and Behavioral Economics at Duke University showed that ignoring someone had a similar impact to ripping up their work in front of them.
In the study they asked students to complete a task and at the end of the task they handed the task on a piece of paper to the experimenter.
In one group the experimenter looked at the paper and said “uh huh” before putting it in the pile. The second group experimenter put the paper in a pile without looking at it and the third group had their work shredded immediately upon completion.
Afterwards the groups were asked how much money they needed to carry on with the tasks.
The people who had work that was shredded needed twice as much money as the group who had received acknowledgement. The group that was ignored had the same results as the shredded group.
So remember every time you ignore an email or phone call it has the same impact as shredding the message in front of them.