Hi Sergio,
below are my finding regarding your question.
Check the below enhancements opened.
https://tools.cisco.com/bugsearch/bug/CSCsz35375/?reffering_site=dumpcr
https://tools.cisco.com/bugsearch/bug/CSCsr65970
For 79XX phone we have this feature available under "Product Specific Configuration"
Handset/Headset Monitor: When handset/headset monitoring is enabled, a supervisor can pick up the inactive handset/headset and hear the call in progress. For example, if an agent is on a call on the headset, a supervisor can hear the other party on the handset. Only the microphoneon the active handset/headset is in use. For example, if an agent is active on the headset, then the headset microphone is active and the handset microphone is disabled.
Unfortunately as it is "Product Specific Configuration" it is not available with 8831 phone.
i would recommend you to contact your accounts manager and open a feature enhancement request for the same.
HTH
Jitender Bhandari