Voice barely audible via the phone speaker (amplifier?)
 

Suddenly I found out that during a phone call the voice of the other person is muffled, barely audible even at the maximal volume. (One can hear that there is something at the other end when being in complete silence)

When using the loudspeaker, everything works correctly.

Is this something with the amplifier? Can I somehow fix it diy?

Re: Voice barely audible via the phone speaker (amplifier?)
 

Dirt in the speaker? Dirt on the contacts?