Your reference to "now" not being able to use mobile data whilst on a call suggests that you have been able to in the past. When did you stop being able to do this?
The obvious cause of this is that your phone is not using VoLTE, the technology underpinning 4G-calling. This would mean your voice calls are using 2G, and explain your symptoms.
On Apple phones, I believe this is enabled by default, but on Android it can be toggled off within your phone's call settings menu. Are you having this problem in one specific location, or is it more general?
