The Problem
Salesforce only allows one SCV Omni-Channel session open. Therefore, it is advisable to open sub-tabs within a Console app in Salesforce rather than new browser tabs or windows. If a call is connected and a new tab is opened, it is recommended to select ‘Cancel’ when prompted to log in to omni-channel on this console. Selecting ‘Continue’ will result in the disconnection of the call, as it terminates the omni-channel session in the previous tab and initiates a new session in the new tab.

If your users are struggling with this issue and keep opening multiple tabs and windows this setting might be useful to enable.
The Fix
Navigate to Salesforce Setup > Omni-Channel Settings > Select ‘Don’t automatically log agents in to Omni-Channel on a new window or tab’.

For more information on this setting, please read Salesforce’s documentation.
Be aware
This is will mean that the first Salesforce tab where they logged in will remain the route of the call. If on a phone call and that window is closed, the call will be terminated.