It is doing exactly what it's supposed to. The iPhone can not handle staying on LTE and being on a call at the same time, so it will go down to HSPA+ to maintain a data connection, while you're on a call. I think it has something to do with the chip, and the antenna configuration.
But it won't go back to lte
But it won't go back to lte
This is the new broken left speaker question.
Sticky!
lol i was just about to write the same thing!