Several conditions could cause a conversation window to become transparent in Skype for Business 2015 . In this situation, the call window is not displayed, and the chat window from which the call is placed becomes transparent. When this problem occurs, the call window and call controls are no longer available.

To resolve this, you can enable this by creating a custom policy entry and add it to the client policy, one of the pre-requisites needed before this would be to or Skype for Business 2015 ensure that the February 5, 2019, update (KB4462135) is installed on the Skype for Business Front-End Server.

Another thing to check before running the client policy command would be to see what Client Policy users are on, this can be done through the Skype Control Panel. If there are multiple Client Policies active, then the below will need to be run for each policy. To enable the custom policy entry, run this command on an elevated Skype Management Shell:

$x = New-CsClientPolicyEntry -Name “EnableTransparentWindowFix” -Value “true”

Set-CsClientPolicy -Identity “<ClientPolicyName>” -PolicyEntry @{Add=$x}

Once that has been completed, if you run the command – Get-CsClientPolicy “ClientPolicyName” and this should then show the new policy entry.