If you unplug a 18 Button set and plug a 24 button set into the jack what happens is the extension will default the key programming for a 24 button set.So whatever is the default key programming in mmc 723 is how the keys will work.Plug the 18 button set back in and it will default to an 18 button template in mmc 723.It sucks and I wish they didn't do this because when you have a system with mixed phone types this is what can happen.I understand why they do it but the end result is like mentioned above you will have to go in to tech programming and reprogram that ext in mmc722.MMC 107 can assign extenders yes BUT the key has to be assigned first.By plugging and unplugging different phone types that is no longer the case.