Are they Cisco switches?
in generic term you will need to find out the following in order to implement QoS for voip purposes:
-what sort of signalling is used between phones and PBX (SIP, H323?).
-what codec do you use? and how many simultanious cross WAN do you want to allow? this will dictate how much bandwidth you need.
stick all your RTP traffic in your highest priority class and use low bandwidth codecs. stick your signalling traffic into highest or second highest group
good luck
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