79-400-0000/B
is a digital phone. It could be the programming. If you can tell me what system you have (818 or a 128) and the version of software I'm sure we can get the problem fixed for ya. To find the ver. turn off the power to the system and pull the MPD card, on that card you'll find a memory cartridge, there's where the version will be. Plug it back in and don't force it. Let us know.