USB headset working for outgoing calls, but not incoming

I have a strange issue. Making outgoing calls with a USB headset on my L5 works fine, audio and mic is picked up by the headset. When answering an incoming call with the headset connected, it either works for a while before it switches over to the internal mic and speaker or just goes with the latter from the beginning of the incoming call.

I have much better audio (and no echo for the receiving end) when I use the headset than while making calls without a headset, so I really want this to work reliably.

Obviously something strange is going on with the audio settings that seemingly without reason switches from a USB headset to the internal counterparts of the phone.

How can I collect the necessary info for any dev to troubleshoot this issue, i.e. which log files and/or settings do I need to check?