Symptoms
Assume that you start a PSTN call in Microsoft Skype for Business 2016. The PSTN user puts the call on hold many times. When you hold the call after the PSTN user takes the call off hold, you may find you can hear the PSTN user, but the PSTN user can't hear you.
Resolution
To fix this issue, install the November 13, 2018, security update for Skype for Business 2016 (KB4461473) and and use one of the following methods:
Method 1
Important: Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case problems occur.
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Exit all Microsoft Office applications.
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Start Registry Editor:
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In Windows 10, go to Start, enter regedit in the Search Windows box, and then select regedit.exe in the search results.
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In Windows 8 and Windows 8.1, move your mouse to the upper-right corner, click Search, type regedit in the search text box, and then click regedit.exe in the search results.
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In Windows 7, click Start, type regedit in the Search programs and files text box, and then click regedit.exe in the search results.
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Locate and then select the following registry subkey:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Lync
Note In this registry path, <xx.0> corresponds to the Skype for Business version (16.0 = Skype for Business 2016).
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On the Edit menu, point to New, and then click DWORD Value.
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Type EnableMediationServerCallFlow, and then press Enter.
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In the Details pane, right-click EnableMediationServerCallFlow, and then click Modify.
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Select Hexadecimal for the base, type 1 in the Value data box, and then click OK.
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Exit Registry Editor.
Method 2
Create an in-band provisioning custom CsClientPolicy entry for CsClientPolicy identities that are assigned to users. For example, run commands that resemble the following in Windows PowerShell:
$a = New-CsClientPolicyEntry policy_name EnableMediationServerCallFlow -Value "True"
Set-CsClientPolicy -Identity policy_name -PolicyEntry @{Add=$a}
Note You must sign out and then sign back in to receive the new provisioning settings.
