Make sure your headset and the mobile device are within the 10m range to each other

Low battery level may cause the disconnection from the mobile

If the battery level is less than 10% charge the headphone

Note: Visibility of the battery level is subjective and may vary brand to brand

a) Set your Mobile Phone to Airplane Mode and then again normal mode. 

Mobile's BT working will back to normal if any unknown error is happening with BT

b)Chances are, you might have an issue with the mobile. You can check if the headset is working well using a different mobile.