I have the iPhone 6 with no problems calling or receiving calls. I did a factory restore to remove my old phone prior to adding my iPhone 6. I've also done the Nav update to version 8 prior to getting my iPhone, so not sure if that is the difference.
Think I heard of a report on this last night where iPhone 6 owners (some obviously) are having problems with these in their cars (not specific to Kia, just bluetooth in general). Also, make sure you guys don't sit with them in your pockets... or bend with them in your pockets, lest you bend your phone lol.
I used the i+cog button on the dash to get into the info/setup screen then did the following:
Press "Restore Factory Settings"
I received a message about my current device being disconnected and verification I wanted to reset my bluetooth settings.
Then I went back into the Phone settings, pressed "Bluetooth Connection" to add my new phone. You may need to forget the former Kia connection from your iPhone in order for the pairing to work correctly.