After a call my ringtone for incoming call goes from full volume back to 1/4 of volume

[AC Question]

Using a Samsung S4 Active I updatet to Lollipop 5.0.1. My ringtone that I use for incoming calls mutes automaticly after I'm ready with a call. It goes from full volume down to 1/4 of the volume so I have to change it to full volume each time. Whay could be the problem and solution for this? I have a few colleagues who have the same issue.