Yes, you can use an ACL such as:
ip access list extended SOFTPHONE-VOICE
permit udp any any range 16384 32767
ip access list extended SOFTPHONE-SIGNALING
permit tcp any any range 2000 2002
This example taken from the QoS SRND here:
http://www.cisco.com/univercd/cc/td/doc/solution/esm/qossrnd.pdf
Hope this helps.
Brandon