A new setting is becoming available for Teams meeting policies to control how dial-in users are handled when joining the meeting.
With this new setting, dial-in users are bypassing the lobby by default.
You can control it by editing the Allow dial-in users to bypass the lobby setting in the meeting policy from the portal
or by using the Teams PowerShell module by turning on or off the AllowPSTNUsersToBypassLobby parameter
NOTE the meeting policy CMDlet’s are available with the Skype for Business PowerShell module (available here https://www.microsoft.com/download/details.aspx?id=39366)
$sfbSession = New-CsOnlineSession
Set-CsTeamsMeetingPolicy –Identity <Teams Meeting Policy> –AllowPSTNUsersToBypassLobby $true or $false