How can I route calls according to the agents' availability?

In the phone settings under Admin -> Phone -> Edit-> Settings, when you move down to "Call Routing" section, you would have two options - "Call idle agent first" or "Call all agents". When the former option is selected the  call would be routed to an agent who has been inactive on the phone channel for the longest amount of time. If the later option is selected the call would ring for all agents irrespective of how many calls they have attended previously.