Revision History; Kx-Tgp500/Kx-Tgp550 Software File Version 12.04/22.04 Or Later; Kx-Tgp500/Kx-Tgp550 Software File Version 12.06/22.06 Or Later; Kx-Tgp500/Kx-Tgp550 Software File Version 12.10/22.10 Or Later - Panasonic KX-TGP500 Administrator's Manual