The 536M expansion module is designed to increase the power & flexibility of the 5i series SIP telephones. Up to three modules can be used with the 55i, 57i or 57i CT telephone to create a powerful, feature rich console option. The 536M shares power & signaling with the phone, eliminating the need for additional wiring. Designed for receptionists, administrative assistants, call center agents, power users, & executives who need to monitor & manage a large volume of calls on a regular basis, the 5i series expansion modules provide an intelligent choice for all enterprise IP environments.