If you are connecting vpn to your company network through a public domain (Internet) i would rather suggest to connect both the ip phone and pix firewall on a switch where you can connect your cable modem.
Since its not gonna be a managed network from end to end the qos markings/config which you do in the end equipments will not be effective since the packets traverse through a unmanaged network.
Also PIX 501 doesn't have the support of QOS classification and marking.
Better connect your voip phone as i said in the beginning and start using with good quality.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...