On the right side of the dialpad, there is an oblong key with a speaker icon and " + - " above and below the icon.
You can adjust the volume of speaker phone, ring tone, and the handset with this button, either through the "menu" or while any of the three functions are in use. 
For instance, if your phone rings and it is too loud or too quiet, you can press + or - to adjust accordingly.
When you are speaking with someone on the handset you can also use the same key(s) to adjust how you hear the caller.
To change your speaker volume, press the clear key with a speaker icon and then adjust up or down. This is also the same process if you have a caller on speaker and need to change the volume.
If you want to mute sounds or a conversation in your office from a caller on speaker, press the key that has an icon with the microphone and a slash through it. Press again to resume your 2-way conversation.
