Chosen Solution
Bought iPhone 6S 4 years back. Still working like a charm. But 2 yrs back, the bottom microphone had some issue. Ie, receivers while on cellular calls cannot hear me well. Shown some error while diagnosed in apple Service centre. But the receiver can hear me well on whatzapp voice calls and other social app voice calls.
The iPhones have more than one microphone and different apps use different ones. For example Siri uses the top microphone. A new charge port assembly should fix your problem.
