Cisco Firewall :: 5610 Can't Connect IP Phone While On Comcast Via VPN?

Can An ISP Filter Traffic within the VPN Tunnel?  Sounds weird but..We have a Avaya IP Office 500 Head end Phone Server. Several 5610 IP Phones.I've setup a PIX 501 to Connect to our ASA 5510. In the Office, going from one Public IP Subnet to the Public IP on the ASA 5510 I'm able to connect up the 5610 IP Phone through the PIX 501 through the ASA 5510 to the IPOffice 500 Server and place calls.I take the same setup home and connect it to my Comcast Internet connection anf it does not work. I can connect a Laptop behind the PIX501 and Connect to the HQ network just fine.  I can see the Phone do a TFTP Transfer to the VM Server, though it stops short can cannot connec to the Call Server.
 
I then gave the unit to 4 other Comcast Users, all of them do not work.I then gave it to a AT&T DSL user, works Great! then another local DSL ISP (Sonic.Net) and it works great.Same hardware, same VPN, Same everything except ISP.Both With Comcast we tried directly to the Cable Modem, or behind a edge router.  PCs connect, Phone does not.The thing I do not understand is If Comcast is filtering something, how can they filter something that is in my VPN Tunnel?