Adjusting the Receive volume (what you hear):
This is controlled using the volume dial of the amplifier base.
On the S10 the dial is labelled "Listen." The S10 also has a dial labelled "Tone" which will change the bass/treble frequency of the incoming sound.
The S12 has a soft rubber volume dial between the headset/handset and mute switches which will increase or decrease your listening volume.
Adjusting the transmit volume (how others hear you):
The S10 and S12 all include two transmit volume adjustments. The TX setting adjusts the transmit level to HI or LOW, and the dial allows you to adjust the sound level within each range.
On the S10, the range setting is located on the left side of the amplifier labelled "Tx Level." The transmit volume dial is on the top of the amplifier labelled "Speak."
On the S12, the TX setting and transmit volume dial are on the right side of the amplifier.
For more information, please see your model's User Guide, attached below.