单项选择题

A.RTP traffic is exchanged between IP phone P1 and IP phone P2. IP phone P1 alerts the CCM of off-hook state. CCM instructs IP phone P1 to provide dialtone. IP phone P1 passes digits to CCM. CCM routes call to PSTN or IP phone P2.
B.CCM instructs IP phone P1 to provide dialtone. IP phone P1 alerts the CCM of off-hook state. IP phone P1 passes digits to CCM. CCM routes call to PSTN or IP phone P2. RTP traffic is exchanged between IP phone P1 and IP phone P2. 
C.CCM instructs IP phone P1 to provide dialtone.IP phone P1 alerts the CCM of off-hook state. IP phone P1 passes digits to CCM. RTP traffic is exchanged between IP phone P1 and IP phone P2. CCM routes call to PSTN or IP phone P2.
D.IP phone P1 alerts the CCM of off-hook state. CCM instructs IP phone P1 to provide dialtone. IP phone P1 passes digits to CCM. CCM routes call to PSTN or IP phone P2. RTP traffic is exchanged between IP phone P1 and IP phone P2.