Abandoned calls are represented in the call metrics tab with a grey slashed arrow, an abandoned call means that the call was hung up by the customer before an agent could answer it. 

A missed or no-answer call is represented by a red slashed arrow, it means that the customer was left unanswered by the agents, no-answered calls are calls that rang for the entire duration of time and were left unattended by your agents. 

NOTE: Abandoned calls are not the fault of the agent, the call was disconnected by the caller before an agent could attend to it. However no-answer calls means that the agent did not pick up the call despite it ringing for the entire duration, here the agent is at fault.