One piece of information that may not be well known is that the Instant Messaging multipoint control unit (IMMCU) is installed by default regardless if the Conferencing functionality is installed or not. This MCU is required for several of the control functions around IM such as the ability to record messages or limit the rate of messages received. As such, the IMMCU is required even for two party “Peer-to-Peer” messaging.
What may also be less well known is that the Lync Mobile client requires the IMMCU and the Web Conferencing service to have Multi-Party Instant Messaging. If conferencing is not installed or the Web Conferencing service is not started a mobile client will see the error message “can’t be reached because of conferencing service issues.” when trying to add a 3rd person (or join an existing multi party IM session).
To verify Lync 2013 Conferencing services are installed, open the Topology Builder, select the Front End Pool where conferencing should be installed, right click and select “Edit Properties…”
Ensure the check box on Conferencing (includes audio, video, and application sharing) is selected. Note: If this service is being installed the Deployment Wizard will need to be run on each Front End server to install those services.
Finally verify the Lync Server Web Conferencing service is running.
Once started on all Front End servers this will now allow mobile clients to participate in Multi-Party IM sessions.