You can attempt to drop an incoming call rather than answering it. Doing this has different effects depending on the call type and other options:
If the call is a hunt group call, dropping the incoming call causes it to be presented to the next agent in the group or follow other hunt group settings (which can include presenting the call to you again).
If the call is a direct call to you, dropping it causes it to:
Go to your forward on no answer destination if set and enabled. See Forwarding Calls.
Else, the call goes to your voicemail if available.
Else, the call continues to ring (drop has no effect).
If the call is returning from being held or parked for too long, you cannot drop it.