I am using the latest version of CUAC Standard on CUCM 12.5(1)su5.
When using CUAC to join two calls on different lines on a 7942 phone, ith Join Across Lines set to "On" on the device configuration, it successful joins the two calls.
When using CUAC to join two calls on different lines on a 7841 or an 8845 phone, with Join And Direct Transfer Policy set to "same line, across line enable" on the device configuration, I am given error message "call control action not successful. Feature is not support on this device"
I know that the join feature was removed from the newer phone types and replaced with Conference > then selecting the Active Call.
I'm wondering with the changed behavior on the newer phones, if CUAC didn't evolve to allow for join across line.