First, this situation should not be happening. I have not used S10+ but I have previously found old phones that have faulty proximity sensor that caused this. So, it might be the case for you.
I would definitely agreed that this is most likely due to the battery. This is an iPhone 6, if you haven’t replaced the battery yet, it is time to do it now.
May be there is an opportunity to have different black plastic housing swappable so it can be used in different countries without those clunky travel adapters.