13. What is the name of the gateway interface/port that faces the Central Office (CO) switch or PBX? A. Foreign Exchange Office (FXO) B. Foreign Exchange Subscriber (FXS) C. Direct Inward Dialing (DID) D. Dual-tone Multi frequency (DTMF) 14. Calculate the number of CCS that is equivalent to 1.5 erlangs. A. 1.5 B. 54 C. 90 D. 150 A. Foreign Exchange Office (FXO) B. Foreign Exchange Subscriber (FXS) C. Direct Inward Dialing (DID) D. Dual-tone Multi frequency (DTMF) 14. Calculate the number of CCS that is equivalent to 1.5 erlangs. A. 1.5 B. 54 C. 90 D. 150.