Polycom VVX 300 Quick User Manual page 19

To connect multiple modules, attach another Cable Connector between the AUX 2
port on the first Expansion Module and the AUX 1 port on the second Expansion
module. You can connect a maximum of three Expansion Modules to one phone.
This device provides an easy transition from traditional PBX features and functionality
into the world of VoIP familiar paper directory cards and intuitive LED visualization
that matches the VVX business media phones.