Cisco IP Phone 7962 with headset -- sound coming from phone

Hello,

Weird issue here. I have a Cisco 7962 with a Plantronics C55 headset attached. The headset is wired through the headset port on the bottom of the phone and the handset is wired in the handset port -- i.e. not in series like most implementations. When I turn on the headset and hit the headset button on the phone, the sound comes through loud and clear on the headset. What irks me is that you can also hear sound coming from the speaker on the phone as well. The handset is hung up yet sound still comes out. It's really low, but it's there. Has anyone else seen this issue?

Any help will be apprecited.

I didn't find the right solution from the Internet.

References:
https://arstechnica.com/civis/viewtopic.php?t=1143350
Teaser video production studio

Thank you.