Why does the sound quality suddenly drop about five minutes into a call?

[Android Central Question]

Hi guys, this is driving me mad..After about five minutes into a call the sound loses all treble, (the crispness), and just sounds very mid range. Sound is fine using phone as mp3 player also fine on bluetooth...A few months ago I factory reset my phone which seemed to cure it but the problem has started again..

 

[B. Diddy]

Welcome to Android Central! Which phone and which carrier? Does the sound quality also do the same thing on headphones?