On my UC520 with few 7961 and 7965 phones I can't conference in extension. After calling and connecting the second extension I press the "Confrn" softkey, get a second call out appearance and dial the number. After the third extension picks up I press "Confrn" softkey again, but instead of joining the two calls I get a message displayed on the originating, first extension: "Cannot complete conference". I used CCA to configure everything and conference calling is configured as best as I can tell.
Any help greatly appreciated.
Config files are attached.