Clear-Com RS-603 (No longer available) - Manual (Page 8)

R S - 6 0 3 / 6 2 3 / R B E L T P A C K S 6
5. Adjust the sidetone and incoming volume levels for both
channels as you and others on the channels speak at the
anticipated levels for the application.
6. To send a call signal, press the button labeled “call” for the
appropriate channel.