Did you setup on the Devices (Phones) and SUBSCRIBE calling search space. The Subscribe calling space relays on/off hook status to/from the other devices in the same PT/CSS. Dont confuse the SUBSCRIBE CSS with the calling CSS, its not the same. (well.. it could be, but I tend to not overlap these two)
Create PT - SYS-SUBSCRIBE-PT
Create CSS SYS-SUBSCIBE-CSS (add the above to it)
Apply this CSS to all the devices, reset/reboot.
I thought there was something else buggy with this I had to do, But I cant recall... (sorry!)