This could be caused by a wifi calling app. Navigate to settings > call settings > (internet call) calling account settings > untick "receive incoming calls" and make sure it says "use internet calling only for internet calls".
Hope this helps
When my S5 was running Android 5.0 I had an option in the wifi menu to enable/disable "automatically connect to passpoint certified wifi hotspots". This option has disappeared after upgrading to 6.0. How can I get it back or enable it? (Without downgrading firmware).
I can find it on mine by opening the memo app, then once you begin creating a new note, hold the region where you are typing, it should give an option the view the clipboard.
There's probably an easier way but it works for me.
Have you tried entering recovery mode and factory resetting the phone from there?
If that didn't work you should install the stock firmware back on the device. Which should remove that infected OS.
Check the pre-installed "Smart Manager" app. Disable the power saver option and see if that helps. Just a thought as it can cause problems with certain apps.