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Let me see if I understood, are you saying that the phone doesn't charge as fast while in use, in this case while on a call? that's normal.
There's nothing wrong with your phone, since you are making a call it will take longer for the battery to recharge, with that said, if you keep the screen on while on a call or if it's a VoIP call it will drain the battery even faster.
I have exactly the same issue - when I plug it in it starts charging and at some point (most probably if it rings or if I make a call - rarely get this issue while charging late at the evening) charging gets very slow and the indicator shows something like 10-14 hours for a full charge. I have to unplug it and plug it back in to get to normal charging rate. Sometimes it takes several attempts to get a full charge - it's very frustrating. Didn't expect this from such a phone...
I installed Ampere app - when charging it shows 1500-1600 mAh. After a call the value drops to 50-300 mAh and stays there. When restarted while still connected to the charger everything gets OK.
I had some troubles replicating the process in safe mode as I can't use the Ampere app to check the charging current but it seems that the problem is non-existent in safe mode. This evening I will try to make a full charge in safe mode after receiving a few calls to see how long will it take to charge.