Illinois is, by statute, a two-party state. However, case law from both the IL Supreme Court and various Illinois appellate courts have declared Illinois a one-party state in the case of private citizens (businesses and plain folks - NOT law enforcement). The reigning consensus is that one-party consensual recording is merely "enhanced note-taking" and since some folks have total recall without recording, how can the other party have any expectation of privacy to a conversation held with another person.
Illinois requires prior consent of all participants to monitor or record a phone conversation. Ill. Rev. Stat. Ch. 38, Sec. 14-2. There is no specific business telephone exception, but in general courts have found extension telephones do not constitute eavesdropping devices. Criminal penalties for unlawful eavesdropping include up to three years' imprisonment or $10,000 in fines and the civil remedy provides for recovery of actual and punitive damages.
In the state of Illinois it is illegal to monitor cordless phones.