Multicast
The phones are able to play a received audio stream in hands-free mode. A list of multicasts can be created for this purpose.
Configuration
Web interface: 
Telephony > Multicast
*Details about the settings are in Multicast
Phone: 
Settings > Telephony > Multicast reception
*Details about the settings are in Multicast
Provisioning: 
*Details about provisioning this function in the Auerswald DokuWiki