You will find 2 arrows below the telephone's screen.

Speaker volume - Press the “Speaker” key, you will hear a dial tone, and adjust the volume with the corresponding arrow. Also can be done if you are on a call placed on speaker and the person is talking too softly or too loudly, you can adjust the volume to hear them at more comfortable level.

Ring volume – While your extension rings you either increase or decrease volume by pressing the corresponding buttons.

Handset volume – Pick up the handset, and either increase or decrease volume by pressing the corresponding buttons. Also can be done if you are on a call, and the person is talking too softly or too loudly, you can adjust the volume while they are speaking.