If Customers hearing on echo on their end and you’ve already tried adjusting the mic volume on the base it may be something called “configurable sidetone”.
Sidetone lets users hear their voices played back in the headset and provides assurance that the phone is working.
This is an admin feature was added to the Unified CM application. It is available for the following phones: 7971G-GE, 7970G, 7961G-GE, 7961G, 7941G-GE, and 7941G phones
How to fix the issue:
Customer will need to contact their phone admin for this step!
Set he Handset/Headset monitor variable in Cisco Call Manager for this phone to 'Disabled'. (This can be found in the configuration Window)