Nokia Lumia :: Volume Level During Call Very Low

I am experimenting a very weird and annoying problem with my Lumia 900. Since last night the volume level during the phone call became suddenly very very low. I can hardly hear the other speaker. Apparently this is only happening during a phone call in normal mode. If I press the loudspeaker button, for example, I can hear the conversation perfectly as usual. Same using the earpiece: I can hear and talk without any problem. Moreover, the audio output appears to be OK outside the phone call itself: when using apps, music, games or any other feature except the phone call, I can't spot anything abnormal in the volume level, everything appears to work properly.

I would like to point out that:1) the volume level during the call is set to the max. level as usual (10/10)2) the master volume of the device, as set though both right-hand buttons, is set as usual to the max. level (30/30).3) I turned off and on the device at least five times since last night. Unfortunately the problem persists4) I made several tests by dialing different numbers, with no success: the volume level is still very very low during a call.5) I did not notice any other problem in the last days nor the device suffered any "catastrophic" event.In other words, I can only hear a speaker during a phone call either keeping the loudspeaker always on or using the earpiece. It's like my call volume were set to 1 or 2, while it is actually set to 10!I am really surprised, I bought my Lumia 900 end of June and I was very, very satisfied so far