Siemens IMPACT Pro M Instructions And Guidance page 11

To increase volume, place your index
finger gently against the volume
control wheel and rotate it upward. If
your IMPACT Pro hearing instrument is
equipped with a warning tone for
volume control limits, once it is heard,
adjusting the volume control further in
that direction will have no effect.
To reduce the volume, rotate the
wheel in the opposite direction.
NOTICE
If your hearing instrument has not
been programmed for manual
control adjustment, the volume
will automatically adjust and
rotating the volume control
wheel will have no effect.