Oppo F1s has a sound issue, when battry is less than 40% the ringtones sounds will be coming directly at the back of the phone not from the bottom where the speaker is located and the sound is like a internal crackling sounds but it only applies pre intalled ringtones not on music.After restarted phone it came back to normal but after a minute it goes the same.It also have a problem with contacts.The number of contacts automatically increased many contacts get duplicated.