You want to classify traffic by setting the ToS byte in the IP header.
Configure DiffServ marking within a policy:
set policy from "Trust" to "Untrust" "voip" "sip.juniper.net" "SIP" permit traffic dscp enable value 18
Traffic classification is used on edge devices to a backbone network, commonly called Customer Edge (CE) routers. Its purpose is to classify traffic flows into service levels for special treatment within the network cloud. Marking refers to overwriting the first three or six bits of the ToS byte within the header of each IP packet. This is different from actually acting on a marking. ScreenOS devices do not act on ToS byte marking, but can pass the current marking or overwrite it. Marking is supported on all NetScreen, SSG, and ISG platforms, and is ASIC-assisted on ASIC platforms, providing marking in line rate. There is no performance penalty with marking. The more granular traffic can be classified, the more control you have on the treatment of traffic within the network cloud. Therefore, edge firewalls are a very good place to do marking because traffic is commonly classified via policies and policies usually can be more complex than ACLs on a router.
Although marking can be totally arbitrary and is up to the service provider of the backbone network, two standards are common: IntServ and DiffServ.
IntServ uses only the first three bits of the ToS byte (also called the Precedence bits, as detailed ...