An electronic real estate lockbox system includes a number of operational features, including facsimile reporting capability, bilateral locking solenoids, improved shackle latching capabilities, improved key-compartment release operation, two-wire bilateral lock-key communications, improved case design, viral lockout list propagation, an update code grace period, packet formatted data transmission with error recovery, FSK audio data downloading, and high security challenge-response authorization procedures. A variety of other operational features are also disclosed.