What can I do when the speaker does not work in telephony apps

[Android Central Question]

Especially I do not know where to ask or report that bug.

What happens:
On my Caterpillar S60 with Android 6 I have the issue, that I cannot hear sound from the phones internal speaker, but only during phone or skype calls. SO I can hear notification sounds, I can listen to music, the speaker also works when I switch the phone app to hands free mode during the phone call. In the not-hands-free-mode however it is just silent. Of course I checked the volume settings and tried to increase or decrease volume. Also it works when I plug in an external headset. However, according to the status icons, the phone does not think that an external headset is plugged without actually being plugged in.
After a clean of the phones cache, it will work for some days and then the same issue reappears.
I do not want to send the phone in, because then I won't have it for 3 to 4 weeks and as it is obviously a software issue, it will probably reappear sooner or later.
What else can I check? Should I and how can I contact the android developers with this issue?
Thank you for your help.

 

[B. Diddy]

Welcome to Android Central! Does this happen in Safe Mode as well? On most Android devices, while powered on, press and hold Power until the Power Off menu appears. Press and hold the Power Off selection until the Safe Mode prompt appears. Tap OK.