Denwa Communications DW-610G User Manual page 75

8.3.5.2 EXT KEY
EXT KEY
has the same usage with the Function key. "In" port connects the phone,
"Out" port connects the next one, if there is only, you don't need for power supply,
if there are more than one, you need supply 5V power for the first one, and use
RJ-45 direct connector.