Swich to 2G
Check this:
Any ways to disable AMR Wideband for now? Can I make Tasker to swith network to 2G when dialing/receiving calls?
Check this:
Actually it could be really easy to do with AutomateIt app. Just create New Rule and place "Call State" as a trigger. Then select trigger's settings to "Active (answered or dialed)" and "All Contact". After that place "Set Mobile Data Action" as a action. Then select action's setting to "Disable Mobile Data". Then just save that rule.
Now your mobile data is disabled when you answer or dial a call. Now you have to make a new rule which enable LTE after call is ended. Place "Call State" as a trigger. Then select trigger's settings to "End call" and "All Contact". Then place "Set Mobile Data Action" as a action and select action's setting to "Enable Mobile Data". Then just save that rule.
Then your mobile data should be disabled when you answer or make a call and enabled when you end call. If it is not working then maybe you have to make third rule that disables mobile data when incoming call arrives. Maybe it is worth a shot.
Ps. It can be better to do with Tasker but my skills aren't that good with Tasker.