To effectively implement priority queuing for VoIP, you should always configure rate-limiting (policing) as well to drop traffic that exceeds a specified level. Priority traffic is not rate limited. Policing is required because the ASAdoes not support traffic

shaping. This means that there is no backpressure on the flows to limit the amount of traffic being sent. Thus, without policing, priority queing really only has an affect if the ASA's interface becomes saturated. Then priority traffic is sent over non-priority

traffic. However, in most cases the ASA will be connected to a switch via 100MB/Full duplex. But the upstream connection to the Internet is much less than 100Mb/Full duplex. So there won't be much queing on the ASA interface, but the upstream device will drop the packets (unless it supports some type of QoS). .

http://www.cisco.com/en/US/docs/security/asa/asa70/configuration/guide/qos.html