Something is making a call on Android OS and System. Do you have any of the following apps installed...
Apps that auto start or run in the background
1. Facebook
2. BlackBerry Messenger
3. 8 Ball Pool
4. Instagram
5. Messenger
6. WeChat
7. Facebook Pages Manager
8. Oovoo Video Call Text & Voice
9. KakaoTalk Free Calls & Text
10. Vine
Apps that are initiated by the user
1. Spotify
2. Amazon Kindle
3. Line Free Calls & Messages
4. Samsung WatchOn Video
5. Snapchat
6. Netflix
7. SoundCloud Music & Audio
8. CleanMaster Speed Booster
9. Tumblr
10. Picsart Photo Studio