Question : VOIP QOS deployment

I am new to QOS so please bear with me.
This is a very basic sketch of my network.

                                   (Site-2-Site VPNs)
Multiple remote sites ------> WAN  <------ 2800 <-----ASA 5510 <----- 2960(G) ---[ LAN with BCM400.

I have started using VOIP between some locations but I am concerned about the implications of not having QOS network wide.
    1. Should I configure QOS on every device (2800, ASA 2960(G) switches etc)?
    2. 20 Remote sites will each have a max of 2 IP/Softphone (Nortel & Cisco) and has 1mb down and    atleast 256kbps upload speeds how would you recommend we allocate bandwidth for VOIP?
       N.B codec: cisco g729 and nortel g711.
If i am missing anything please help!


If I have to configured QOS on each device  

Answer : VOIP QOS deployment

No you can still have the phones on a different VLAN even if the computers are daisy chained off the back of them. You just set the switch to enable 802.1q trunking and set the voice VLAN number to be permitted over the trunk. Then on the phones you configure them to perform VLAN tagging. Then all voice traffic is sent over the VLAN while normal data is untagged and so goes over the default VLAN the port on the switch is configured to use.
You certenly dont have to keep voice and data separate in this fashion. Often people use voip clients on their pc's or have some management panel running on their pc so the networks have to be combined and often it is easier to run it all over a single network anyway.

I would setup a test. Get a phone and daisy chain a pc off the back of it. Make a call and then try and copy a very large file to and from the pc and see if you can hear anything during the phone call. If possible try it will a ftp file transfer to the server with a gigabit network connection as this will be worst case. Windows file sharing is far less efficient than ftp so if ftp does not cause a problem copying a file in windows shouldnt either.

Configuring a switch to use QOS is normally fairly easy. It really easy if you dedicate a VLAN for voice as cisco switches have a preset configuration for this that you practically just need to turn on.
If you are sharing the voice and data you need to configure the switch to trust the difserv/tos (QOS indication flags in the packet headers) that the phones set automatically so it doesnt really make much difference.