Press Program button - press 00 - press 1
The phone will display your volume level on a display if available and audibly ring at that level, which you can adjust using the arrows at the bottom of the key pad.
If this does not work, try unplugging the phone for a few moments then plug back in making sure the power is completely off the phone, this will often reset it and take care of the issue. If this does not work swap the phone.