The left edge of the phone houses two buttons for volume adjustment, while the right side is where the power / standby button is found. (brighthand.com)
Depending on the model you will see lots of different level of volume adjustment. (maternityfolks.com)
They fit little heads and have a separate volume adjustment on the cord, so you can be certain the noise levels aren't too loud for their ears. (havebabywilltravel.com)