iPhone 7 Plus call quality issue

An old thread had literally thousands of people complaining that when they called people with their iPhone 7 / 7 plus their voice is cutting out and the other person can’t hear them. The thread is now closed but I was wondering if this was ever resolved? Recently took the plunge to buy a second hand iPhone 7 Plus and get some get some good pics of our newborn and we have the aforementioned issue. The sound is fine when using voice recorder but as soon as you make a call, the quality is awful. Admit I should have got a guarantee but at the moment it looks like we’ll have to throw the phone away at a £200 loss. We replaced the speaker but made no difference. Have tried all the things I see recommended. Was a full solution ever found?