You need to enable UDP jitter.
UDP Jitter Operation Vs RTP VoIP Operation
UDP Jitter is an operation that sends a stream of UDP test packets specially crafted for performance measurement. There is another operation called "RTP VoIP". There are fundamental differences between the two:
RTP VoIP sends a real RTP stream, while UDP Jitter sends a simulated stream. Both looks the same from the outside, but RTP VoIP packets contains a real RTP stream.
RTP VoIP uses and requires at lease one hardware codec to run. It needs DSP resources and voice interfaces to run on.
RTP VoIP will compute the MOS score based on the delay and number of DSP frames lost. UDP Jitter provides IP-based metrics, while RTP VoIP provides DSP-based metrics.
RTP VoIP is an operation that is very specific for VoIP, and UDP Jitter is more generic.
RTP VoIP is not designed to be deployed at large scale, while UDP Jitter is.
It is recommended to use UDP Jitter as much as possible, and it will be enough for the large majority of the cases. When more detailed information is needed, or for troubleshooting purposes, an RTP VoIP operation can be triggered on demand at specific time and location. RTP VoIP is not meant to replace UDP Jitter.
Courtesy: Info sourced from here:
http://docwiki.cisco.com/wiki/IOS_IP_SLAs_UDP_Jitter_Operation_Technical_Analysis