We recently bought some 78xx IP phone and we found the same problem: when I try to transfer a call to another phone (either an internal or external phone) I cannot cancel the transfer, neither by pressing the line button nor the cancel softkey.
Once the cancel button is pressed the call is automatically diverted from the originating caller to the third phone, and is terminated on 78xx phone.
We already installed the latest CUCM version (18.104.22.16800) and phone firmware (sip78xx.10-1-1-9) but the issue is still present.
These are the paths to get to each CCX logs through CLI. They may be helpful if you are having issues accessing RTMT or downloading logs through it.
If you want to download them you have to prefix "file get " and you can add one of the options (re...