Those of you with a new iPhone 7 might notice that the voice quality can sound like a poor Skype call despite being on LTE networks. This drove me crazy at first because my phone felt like a downgrade from my iPhone 6, but there is a simple way to improve it.
Go to SETTINGS > CELLULAR > CELLULAR DATA OPTIONS > ENABLE LTE > Select DATA ONLY.
It ships with VOICE & DATA selected and the quality is awful unless you live in an area with a very strong LTE signal.
Hope this works for you.