The Single Line Interface Board (SLIB4) ( Part # 3533-00 ) provides the interface to four 2500-type telephones. The SLIB signals interface with industry-standard ringers and message waiting lights. In addition, one LED is mounted on the PCB to indicate the in use state of the connected telephones. It will turn on when one or more ports are busy. The optional 4-port SLIB4 may be added to the BKSU or EKSU to expand the number of single lines stations, and can support an MSGU48 board in a daughter-board-type arrangement.