A system for detecting inmate to inmate conference calls in a correctional facility is disclosed herein. The system includes a database and a conference call detection server, wherein the conference call detection server is configured to monitor a plurality of inmate communications, convert an audio signal of each inmate communication to a frequency domain signal, identify frequency data comprising one or more frequency peaks and corresponding frequency values in the frequency domain signal for each inmate communication, generate a record comprising the frequency data for each inmate communication, resulting in a plurality of records, store the plurality of records in the database, detect an inmate to inmate conference call by matching a frequency subset of a new inmate communication with frequency data in a detected record in the database, and verify the inmate to inmate conference call by matching audio with voice biometric samples.