Polycom SoundPoint IP 335 - Getting Started with Your SoundPoint IP 335 Phone; Parts List; Assembling Your SoundPoint IP 335 Phone; Connecting Network and Power Source

Adjusting the Receiver Volume
 To adjust the receiver volume, press the – or + keys
during the call.
 The phone will keep this receiver volume setting after
you hang up, so you don’t have to reset it on each
call.
 SUTUS recommends that you do not increase the
receiver volume to maximum because it can cause an
echo.