The following guide will help you to make Plantronics headset buttons work.
sudo evtest
less /usr/src/linux-headers-$(uname -r)/include/uapi/linux/input.h
lsusb | grep -i plantronics
ACTION!="add|change", GOTO="xorg_plantronics_end" KERNEL!="event*", GOTO="xorg_plantronics_end" ENV{ID_VENDOR_ID}=="047f", ENV{ID_MODEL_ID}=="c013", ENV{ID_INPUT_KEY}="1" ENV{ID_VENDOR_ID}=="047f", ENV{ID_MODEL_ID}=="c013", RUN+="keymap $name 0xFFA000B1 volumeup 0xFFA000B2 volumedown 0xffa000b5 micmute 0xffa000b7 phone 0xffa000ab unknown" LABEL="xorg_plantronics_end"
sudo udevadm control --reload
ACTION!="add|change", GOTO="xorg_plantronics_end" KERNEL!="event*", GOTO="xorg_plantronics_end" SUBSYSTEMS=="usb", ENV{ID_BUS}=="", IMPORT{builtin}="usb_id" ENV{ID_VENDOR_ID}=="047f", ENV{ID_MODEL_ID}=="c013", ENV{ID_INPUT_KEY}="1", ENV{ID_INPUT_KEYBOARD}="1" LABEL="xorg_plantronics_end"
# Plantronics .Audio 648 USB keyboard:usb:v047FpC013* KEYBOARD_KEY_ffa000b1=volumeup KEYBOARD_KEY_ffa000b2=volumedown KEYBOARD_KEY_ffa000b5=micmute KEYBOARD_KEY_ffa000b7=phone KEYBOARD_KEY_ffa000ab=unknown
sudo udevadm hwdb --update sudo udevadm control --reload