Since mine isn't here yet, I guess someone else can test this. It's from the Joe's Tech Review website and it's for a different model, but it might work.
Z1 smart watch phone Engineering code. (Bas Advalvas)
INCREASE CALL VOLUME: found it!
Finally found it. And it's easy! Call sound value is too low on the Z1, 1 of it's main drawbacks.
■Start the phone dialer and dial: *#*#8787#*#* This will start Engineer Mode on de the Z1.
■Select Audio, then Normal Mode. Choose the 'type' Speech, change the value 6 into 0 (that's the loudest) and press Set.
■Press the Back button once. Now select LoudSpeaker Mode. Choose the 'type' Speech, change the value 24 (?) into 10 (on my Z1, the value 0 made the sound very soft, probably because of the system that prevents feedbacklooping from speaker to microphone (how you call this?)) and press Set. Press the Back Button a few times to leave Engineer Mode.
That's all! The standard call volume increases a bit (40%?), but the handsfree volume (which is otherwise about the same as the standard) inceases a lot (150%?) AND the annoying interference sound is now gone!!!