Cisco phones are often the odd ones in a group of other phone brands when it comes to compatibility. Don't get me wrong, Cisco phones are great with headsets, they just have different settings that need to be changed, and here they are!
1 - connected through Handset jack
4 - connected through the Headset jack
A - connected through Handset jack
C - connected through Headset jack
Plantronics CS500 Series (DISCONTINUED)
“A” - connected through Handset jack
“D” or “E” - connected through Headset jack
Plantronics CS Series Wireless Headsets (DISCONTINUED)
#1 - connected through Handset jack
#4 - connected through Headset jack
GN Netcom Wireless Headsets (DISCONTINUED)
“A” or “G” - connected through Handset jack
“B” - connected through Headset jack
AT&T Marathon (DISCONTINUED)
A - connected through the Headset jack
D - connected through Handset jack