There are a wide array of reasons for this issue. The most common is for you to go to system setting and adjust the voice enhancement setting on or off. This usually fixes the issue. If not check for software updates released by LG to fix sound issues. If no updates are available then the antenna may not be receiving proper reception. To fix this issue try relocating to an area with better cell reception.

Try turning on the battery saver mode on the phone. If this does not work try turning off the location tracker as this also drains a lot of battery. If still facing the same issue try turning off apps that drain a lot of battery such as facebook, or twitter, or it might just be a faulty battery, in which case you need to replace the battery using the LG G2 VS980 Battery Replacement Guide.