The iPhone 6 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.