I live in The Netherlands, and I am using a 16 Mbit down/1.5 Mbit up cable connection.
I have my computers and VoIP phone connected to a Linksys WRT54G v2.2 using the Open Source firmware "Tomato" (
Firmware | polarcloud.com)
This firmware comes with limited, but pretty nice QoS functionality. My experience with this is that I can use my connection for 100%, without my VoIP service being disturbed in any way.
My ISP is also offering a "Voice over Cable" service, which is just plain VoIP being offered over cable. My parents are using this service, which is also pretty nice to use. All QoS for this service is being handled from inside my cable modem (Motorola SURFboard).
I think QoS from your ISP side is nice, but I don't think they should let you pay any extra for that. If any of you have a WRT54G(L) router, I would recommend trying the Tomato firmware.