You can only increase/decrease the volume when there is a call already in progress by pressing the directional arrow keys. if you already tried this, then there could be something obstructing the speaker hole you can try cleaning it out and see if it helps.
the speaker could be going bad?
if you have access to parts or tools, you can try to change the speaker out to this one:
also while you have it apart, you can also check to see if there is dirt/debris covering it or its damaged/defective.
if you aren't a handy man, you can take it to a Fli Market and see if they can do it for you.