Your phone lets you handle multiple calls at the same time. Whilst you can only have one call connected (talking) at any time, you can have other calls on hold.
Each current call and any new alerting calls will appear on one of the phone's appearance buttons. The system doesn't treat you as busy to further calls until all your available call appearance buttons are in use.
•Diverting a Call to Voicemail
•Ignoring/Silencing an Alerting Call