An engineer has a SIP TRUNK configured between CUCM and VCS cluster. When a call is made from a Telephone EX60 that is registered to the VCS to an IP PHONE 9971 that is registered on the CUCM, it rings. But upon picking up the call, a busy tone is heard. What should be checked to resolve this issue?

A.	CUCM zone on the VCS
B.	SIP trunk registration
C.	authentication on the SIP trunk
D.	SIP trunk and phone region settings