Please check the socket of the device that you are using for any dirt or debris. It is the tip of the plug that controls the microphone and remote and any dirt or debris will prevent clear communication between the two devices. It may be necessary to use a needle to clean out the socket.
It is also helpful to test the headphones with an iPhone (and a different iPhone than yours if possible). This will allow you to determine if the issue is with the headphones themselves or if the issue is between the headphones and your device.
Make sure your phone uses the same standard as Marshall Headphones, CTIA.
If you have tried all of the above and are still experiencing an issue, please contact firstname.lastname@example.org.