I have the same effect. In my case, with version 14.0.98 (BETA) and Xperia Z3 using Android 6.0.1.
The App is working but I don't have any icon associated with 3CX Phone in the notification area.
Actually we are going to release a final build that removes it.
No other app has this notification icon and it is a battery waster. So this is actually removed on the build we are currently testing.
I sort of like the icon there as it reminds me that the App is still running. I don't normally keep it on all the time, neither do I have data on all the time, I just enable as needed. I do understand the battery concern though.
That icon is another reason to cause more battery usage. Apps on smartphones (unless they are GPS or Audio related like music players) should not have running processes.
Apps should responsibly go to background mode and re-awoken on demand by push. Actually you have an option Exit and Exit without push. You should press Exit so you enable push and app will be launched only when needed.
We uploaded another beta today that answers calls from lock screen.