Digital extensionone of the top of the range telephones in the siemens optipoint 500 range is the siemens optipoint 500 advance system telephone it is compatible with the siemens hicom and hipath telephone systems but has to be connected to a digital extension set up for this type of telephone if it is plugged into a socket not set up for a siemens optipoint 500 telephone it will not workoptipoint with 19 programmable keysthe siemens optipoint 500 advance system phone has 19 programmable keys