A collaboration engineer has been asked to implement secure real-time protocol between a Cisco Unified CM and its SIP gateway. Which option is a consideration for this implementation? 

Answer options:

A. only T.38 and Cisco fax protocol are supported
B. SIP require the all time be sent in GMT
C. Call hold RE-INVITE is not supported
D. SRTP is supported only in cisco IOS 15.x and higher

B