Does anyone know who it is calling

It automatically called and say "goodbye". WTF

Does anybody knows who are those people? They don't leave message, I call them back and a recording says:  
Verizon wireless, you call can not be completed as dialed etc....

Every day it is a different number but same irritation. No one says anything or if u leave it to go to voice mail they don't leave a message. What the hell is their purpose?

Tired of these people calling me! I am on the "Do not call list!" They have been informed of this and to remove my number and to stop caling me, they wait a week and start again!