the voice mail system is embodied on a card designed to be inserted into the pbx cabinet and communicate with the pbx processor via the backplane of the system . the vxc card includes a dual port ram for providing two way communication between the vxc and the pbx . through the allocation of memory areas , and control commands corresponding to the respective allocated areas , the pbx processor and vxc can communicate via the dual port ram interface over the pbx backplane . the use of the dual port ram communication over the pbx backplane eliminates the need for asynchronous rs - 232 signaling to establish control and implement voice mail functions to a pbx system .