1. Check is QoS enabled at the switch, and if not - enable it globally

show mls qos
mls qos

2. Check COS-DSCP maps, and if not correct - change it.

show mls qos maps | beg Cos-

if the question tells to use CS3, then 3rd CoS 3 should beconfigured with DSCP 24, it it says AF31, then with DSCP 26.

mls qos map cos 0 8 16 24 32 46 48 56

3. If the question says you cannot use switch to do remarking, then you must just trust the COS values:

On the Phone/PC port

4. If the question says you must remark signaling traffic with DSCP AF31, then we can use "softphone" model, and modify the config to fit our needs:

int fa0/1
   auto qos voip cisco-soft
!
! The following config automatically generated:

===
 srr-queue bandwidth share 10 10 60 20
 priority-queue out
 auto qos voip cisco-softphone
 spanning-tree portfast
 service-policy input AutoQoS-Police-SoftPhone
!
class-map match-all AutoQoS-VoIP-RTP-Trust
 match ip dscp ef
class-map match-all AutoQoS-VoIP-Control-Trust
 match ip dscp cs3  af31
!
policy-map AutoQoS-Police-SoftPhone
 class AutoQoS-VoIP-RTP-Trust
  set dscp ef
  police 320000 8000 exceed-action policed-dscp-transmit
 class AutoQoS-VoIP-Control-Trust
  set dscp cs3
  police 32000 8000 exceed-action policed-dscp-transmit
===