ACP and VoIP audio don't bridge, and parties can't hear one another in Skype for Business Online

Open domain federation is required for Skype for Business Online meetings that involve an ACP. This is true because the ACP joins the Skype for Business Online conference as an anonymous user. If domain federation isn't configured to use the Allow federation with all domains except those I block setting, the audio won't bridge between the ACP and the Skype for Business Online Voice over IP (VoIP) users.

Note The bridge connection between Skype for Business Online and an ACP is enabled when a Lync client joins the conference by using VOIP audio. The Call me at option and connection of public switched telephone network (PSTN) dial-in users in won’t be possible before the first presenter-level Lync user joins the call by using VOIP audio.

To resolve this issue, take one of the following actions, as appropriate for your situation:

• Set the Skype for Business Online domain federation option in the Skype for Business Admin Center to Allow federation with all domains except those I block.
• Hang up, and then dial in to the conference again to verify that the correct conference ID is entered.
• Users who are connected through Skype for Business Online should verify that ACP participants are displayed in the attendee roster and that the participants have the correct number.
• Users who are connected through Skype for Business Online should verify that they have a solid network connection and that they don't receive errors about decreased network or audio quality.
• Make sure that the toll number and toll-free number are entered without any punctuation or parentheses when dial-in conferencing for a Skype for Business Online user is provisioned.
• Make sure that the meeting organizer’s Skype for Business Online account and the ACP account are configured in the same geographic region. For example, if your Office 365 organization is based in North America, you must use an ACP account that's configured for North America and can't use an ACP account that's configured for the United Kingdom. You can contact your ACP to verify that your account was created in the appropriate region.
• In a hybrid scenario, run the following PowerShell cmdlet:

   Get-CsTenantFederationConfiguration -SharedSipAddressSpace $true  

  Then make sure that the domains of the ACP are added as allowed domains in Lync Server. And finally, provide your external edge fully qualified domain name (FQDN) to the ACP so that they can do the same.
  
  For more information, go to the following Microsoft websites:
  • 2829500 "Move-CsUser : HostedMigration fault" error message when you move users to Skype for Business Online from an on-premises Microsoft Lync Server
  • Configure federation for an audio conferencing provider