If you are using an Audio Conference Provider (ACP) - and you experienced the following: after you dial in to a Microsoft Lync Online conference by using your ACP access number, you cannot hear other users who are connected to the Lync Online conference through their computers, and vice versa - one thing to check is if you've configured your Lync Online service to Open domain federation.
Open domain federation is required for Lync Online meetings that involve an ACP. This is true because the ACP joins the Lync Online conference as an anonymous user. If domain federation is not configured to use the Allow federation with all domains except those I block setting, the audio will not bridge between the ACP and the Lync Online VoIP users.To resolve this issue, follow these steps:
We use Communique Conferencing for Lync Online audio conferencing