The iPhone 6s Plus has at least 3 different microphones in different places for the phone for different purposes

• The bottom of the phone, the left speaker grill is the mic for voice calls
• One near your proximity sensor, used for voice commands for Siri
• The one in the back of your in the camera square. There is a tiny hole for the mic to pick up audio input while recording a video

Now your microphone issue may be caused by any of the one or all 3 microphones not working. There are some DIY (Do it yourself) fixes to try before you come to a decision

Make sure if you use a protective cover for your phone. It doesn’t obstruct any speaker grills or mic holes. Clear the phone to check and make sure there isn’t any debris in there

Try recording your voice in a voice memo to make sure the mic works go to: Settings > Privacy > Microphone and make sure the app is listed there for it to use the microphone