Well in looking at reviews of the HBH 660 online I found a review that said the HBH has two volume controls. If you use the rocker during a call it increases the volume. If you use the rocker when you are not on a call it adjusts the volume of the ringing and the beep tone.

"The delay on restoring the link to the phone when making or receiving a call is negligible, though the beeps emitted at all stages of the process are irksome. Fortunately, the volume control can separately adjust the beep volume and the call volume, though the latter only when actually engaged in a call."