单项选择题
A.voice gateways with analog interfaces need a gatekeeper to convert analog signals into digital format before voice is encapsulated into IP packets
B.voice gateways need a gatekeeper to interconnect the VoIP networks the PSTN network
C.voice gateways with Cisco Unified CallManager Express （CME） can permanently act as a call agent for IP phones
D.voice gateways equipped with a DSP module can take the role of the call agent during WAN failure.