That is the bottom microphone working intermittently. Firstly I'd open the phone up and check for liquid damage inside the phone before replacing any parts first.
If you do find liquid damage on any connectors or the logic board components use 99% isopropyl alcohol (usually with a brush) to brush off the corrosion and liquid residue.
If the microphone still has intermittent issues after that then you will need to replace the charging dock connector which has the main microphone used for calls on it.
Guide to replace lightning jack assembly:
iPhone 6 Lightning Connector Assembly Replacement
You can buy the part from here:
iPhone 6 Lightning Connector and Headphone Jack