Set-CsMeetingConfiguration

Get-CsMeetingConfiguration | Where-Object {$_.AdmitAnonymousUsersByDefault -eq $False} | Set-CsMeetingConfiguration -PstnCallersBypassLobby $True

(source 

https://technet.microsoft.com/en-us/library/gg398648.aspx

Example 3, all the meeting configuration settings that do not allow the default admission of anonymous users are modified. To perform this task, the command first calls the Get-CsMeetingConfiguration cmdlet to return a collection of all the meeting configuration settings currently in use. This collection is then piped to the Where-Object cmdlet, which picks out only those settings where the AdmitAnonymousUsersByDefault property is equal to False. In turn, this filtered collection is piped to the Set-CsMeetingConfiguration cmdlet, which takes each item in the collection and sets the PstnCallersBypassLobby property to True.

Set-CsMeetingConfiguration enables you to modify the meeting configuration settings currently in use in your organization. Meeting configuration settings help dictate the type of meetings (also called conferences) that users can create, and also control how (or even if) anonymous users and dial-in conferencing users can join these meetings