Ringtone volume to the maximum, and which then lowers by itself

It happens at least 2 - 3 times a day that the volume of the ringtone that I put to the maximum lowers a lot, and it therefore happens that sometimes I do not hear the phone ring, I contacted the assistance service several times via chat, telling me to make several attempts ... but none of these solved the problem ... Did any of you have the same problem as me ???