Skype for Business should use Proxy Server

Set-Skype4BProxyUsage.ps1

<#
		.SYNOPSIS
		Skype for Business should use Proxy Server
	
		.DESCRIPTION
		Skype for Business should use Proxy Server to sign in instead of trying a direct connection.
		Works with Skype for Business 2015 and 2016 and should work with Lync 1013 as well.
	
		.EXAMPLE
		PS C:\> .\Set-Skype4BProxyUsage.ps1
	
		.NOTES
		Please note: This is a per user setting!

		SIP is not used between Clients (aka P2P or Cleint to Client).
		It wil be used between Client and Server and/or Server and Server.
		Media Bypass will be used for RTP media between Clients (when possible)

		.LINK
		https://support.microsoft.com/en-us/help/3207112/skype-for-business-should-use-proxy-server-to-sign-in-instead-of-tryin

		.LINK
		https://blogs.technet.microsoft.com/uclobby/2016/12/08/enabling-lyncsfb-client-to-use-proxy-server-for-sip-traffic-instead-of-trying-direct-connection/
#>
[CmdletBinding()]
param ()

$parameters = @{
	Path          = 'HKCU:\Software\Microsoft\UCCPlatform\Lync'
	Name          = 'EnableDetectProxyForAllConnections'
	PropertyType  = 'DWORD'
	Value         = '1'
	Force         = $true
	ErrorAction   = 'Stop'
	WarningAction = 'SilentlyContinue'
}

try
{
	$null = (New-ItemProperty @parameters)
	Write-Verbose -Message 'New value set.'
}
catch
{
	try
	{
		$null = (Set-ItemProperty @parameters)
		Write-Verbose -Message 'Existing value modified.'
	}
	catch
	{
		Write-Warning -Message 'Unable to create/set the value.'
	}
}