Call centre

I keep on receiving calls from call centre agents. When i asnwer no one says anything and a few seconds later i get an sms infoeming me that they tried to call me i should call them back. I dont understand why is it that someone will call me then not say anything to me then have a nerve to sent me an sms telling me they tried to call me i should contact them back. If your call centre agents wont say anything they might as well not call. I am liable for all these calls not them. Train the agents correctly!