I have a acer LiquidZest4G and when someone phones me I can't hear them.. how do I change the volume

Step 1- Go to Google Play store and install the Engineering Mode MTK Shortcut free app

https://play.google.com/store/apps/details?id=com.Go.EngModeMtkShortcut&hl=en

Step 2- Launch it in order to enter engineer mode (MTK).

Step 3- Swipe towards right and go to hardware testing (tab titles on top).

Step 4- Select Audio option (first option).

Step 5- Go to "Normal mode" (first option).

Step 6-
You'll see 4 options there..  “Type” , "Level", "Value is", and “Max Vol” 

For Type you'll have probably these options (or close to it): Sip, Mic, Sph, Sph2, Sid, Media

I believe the standard phone call speaker is "Sph"

If you select "Sph" under Type, and then for "Level" select Level 6... What is the "Value" it shows ?

Range is 0~160 ?  and value is set to what ? 

Take note of the value (just in case - to be able to put it back).  Then you can increase the value slightly (+5 is a good start).  Do not go crazy jumps (you don't want to damage the speaker).  On my Zest Plus, it is set to 144 and it sounds just fine (but every phone is different so take that value with a grain of salt).  Make sure to press "SET" to lock in the value.  I believe both "Sph" and "Sph2" need to be set identical. So you will need to select type "Sph2", "Level 6" again, and set the same value, and press "SET".

Now try it again.

If you find a nice high value, you may want to adjust the other levels (Level 4, Level 5...), in order to have a linear inceease/decrease when you adjust in call volume (so the last click doesn't jump from barely audible, to super loud).  (I'm assuming you DID try adjust volume WHILE in a call)

PS: Don't start playing in other values unless you know what you are doing.  Setting whacked values in some settings can cause damage to your phone.