[Davis92] A. Davis and S. Goyal. Knowledge-based Management of Cellular Clone Fraud, Third IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, 1992. [This paper describes how some cellular providers counter different sorts of attempts at toll fraud through logical intrusion detection. Specific techniques include looking for known patterns of misuse including, high call volume, high percentage of toll calls, calls to suspicious numbers or locations, calls from suspicious locations, calls at unusual times of day.]