To determine which agents are allowed or preferred to receive inbound calls, you can create skills on the Skills tab. Use the Create new-button (3) to make a new skill; simply give the skill a name (1) and an optional description (2) so your fellow admins know what it's meant for.
After you've created some skills, you can assign them to your agents.
In this overview you'll find all agents (1) and which skills they have assigned (2).
To assign skills to an agent, click the edit (3) button behind the agent's name. A pop-up menu will appear where you can see all created skills. To assign the skill, use the check-button in front of the skill. You can then use the point system (4) to indicate which agent is preferred for an incoming call (the higher the amount of points, the more priority the agent has over the other agent with less skill points).
If two agents have the same amount of skill points, the agent who has been idle the longest will get the call first.