Unknown Incidents are a regular occurrance in a 911 center. I would break this into two separate types of calls:
911 Hangup – Could be accidental (False Call) or lead to an incident such as Domestic Dispute, Burglary, etc.
Unknown Problem – This type of call would be when someone calls 911 and either cannot be understood, is uncooperative, or it's just an open line where something suspicious is heard in the background, etc. This kind of call could turn out to be false as well however could lead to a larger variety type of incident base from law enforcement, residential fires, medical calls, etc.