i got same issue with my 2 of IPHONE 7 128gb.
after update.,
while calling speaker grey out, some time works with bluetooth connected else .
Speaker sound working fine with videos and all.
Apple service centre just know how to restore , that customer can do him self.
Saying hardware problem, :-(.
trust now disappearing on product , specially service always rude and blaming.
after 8 years, thinking to move with other product , because of rude reply and not accepting fault at all