Link to Cisco LFI Configuration.
http://www.cisco.com/en/US/docs/ios/12_2/qos/configuration/guide/qcflfi.html
Your VoIP network is a little problematic to say the least. VoIP calls sound choppy and users are complaining all over the place. You look into the datastreams and it seems large data frames are preventing VoIP packets to get to their destination in time. You can't upgrade the links but you have read about link fragmentation and interleaving...let's chop em down!
c3640-jk9s-mz.124-16.bin

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Link to Cisco LFI Configuration.
http://www.cisco.com/en/US/docs/ios/12_2/qos/configuration/guide/qcflfi.html
here is the vedio link
http://www.youtube.com/watch?v=odFOoefgL4M
It's pretty much the same but the video is how to configure multilink PPP over frame-relay, it's a different lab.