Hi @Dean.Smith,
The silent call monitoring is a feature of Webex Contact Center (WxCC). It is not part of Webex Calling supported features.
The other option is Barge-In feature which is available in Webex Calling without the need of WxCC. With Barge-In feature user can answer someone else's phone call or barge in on a call they've already answered. Please remember that this feature creates a conference call so you can not monitor silently until and unless you put yourself on mute. Please check out below video for more details:
https://callinghelp.webex.com/barge-in-feature-video-for-administrators/
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Thanks, Vaijanath S.