Make sure the headphones are securely plugged into the headphone jack on the phone
Applies to:
Verify that the headphone jack is free of debris. Use a torch to see if any lint or other debris may be hindering the connection of the headset to the phone
Remove and reseat the input cable from the phone. The input cable may have come loose or may not be inserted properly
Try using the headset on a different phone. It is possible that the phone is not compatible with the headset