class-map match-any voice
 match dscp ef
!
policy-map strip-dscp
 class class-default
  set dscp default
!
policy-map set-dscp-ef
 class class-default
  set dscp ef
!
policy-map voice-priority
  class voice
   priority percent 100
  class class-default
   fair-queue
   random-detect dscp-based
interface Serial0
 ip address 1.2.3.2 255.255.255.252
 max-reserved-bandwidth 100
 service-policy input strip-dscp
 service-policy output voice-priority
!
interface Serial1
 ip address 2.3.4.2 255.255.255.252
 max-reserved-bandwidth 100
 service-policy input set-dscp-ef
 service-policy output voice-priority
!
interface Serial3
 ip address 2.3.5.2 255.255.255.252
 max-reserved-bandwidth 100
 service-policy output voice-priority