Digital extensionwhen connecting the siemens optipoint 500 entry system telephones they need to be plugged into a digital telephone extension specifically designed for an optipoint 500 digital telephone if you try and connect them to another type of extension or telephone line then the telephone will not work8 programmable keysthe siemens optipoint 500 entry system telephone has 8 programmable keys that the user can set to there preference it stops the need for multiple button presses to do t