reasons not to deploy QOS for VOIP on our Cisco 1800 router from our ISP

We have just deployed a new VOIP telephone system from Avaya, we are using the internet to route our calls over a 2meg lease line.  Voice and data traffic is kept separate on our internal network.  All traffic is then routed through a Cisco 1800 router.

I asked our ISP who manage the router for us to deploy QOS to give preference to the VOIP traffic.

I have been sent the following e-mail explaining why we should not use QOS for prioritizing the VOIP traffic.  Not been an expert in this area I would like some assistance with this.  Is what they are saying correct or are they pulling a fast one?  

HERE IS THE EMAIL

I take your point regarding the QOS feature that you would like to enable on the router to prioritize your voice packets. Lets take a step back and understand what QOS can and cannot do in your scenario.

Quality of service is the ability to provide different priority to different applications, users, or data flows, or to guarantee a certain level of performance to a data flow. Quality of service guarantees are important if the network capacity is insufficient, especially for real-time streaming multimedia applications such as voice over IP.

By implementing QOS on your router it may guarantee that any outgoing voip packet may be prioritized but lets not forget that the VoIP call is bi-directional application . That would entail two points that your QOS mechanism needs to be bi-directional and your sender (other party you are calling) should also understand, accept, and respond with same QOS labeling. This would work fine if you and the other party had a dedicated link or MPLS network to share data and voice and voice is prioritized on the CPE at both ends ensuring same level of bandwidth and labeling.

But how does QOS behave on internet?

When a collection of related packets (VoIP in your scenario) is routed through the Internet, different packets may take different routes, each resulting in a different delay. The result is that the packets arrive in a different order than they were sent. It is key to understand that VOIP packet delivery needs to be contiguous

Also to carry the QOS the forwarding routers on the internet will need to understand that QOS label implemented on your CPE. With internet made up by billions of users and millions of routers it is not possible to have QOS enabled on all of them.

Just to drill in to QOS , there are multiple QOS mechanisms which can come in to play such as Classification techniques DSCP, NBAR; Congestion avoidance techniques WRED; traffic conditioners policing, shaping; congestion management techniques PQ, CQ, WFQ, CBWFQ, LLQ etc .

One configuring QOS should not only understand how these mechanisms work but also understand how they would behave with customers allocated internet bandwidth, other data on network, VoIP codec type, number of concurrent calls etc. Due to such complications and the inability to support VoIP over the internet path we have concluded that it is inappropriate to configure and support QOS in your scenario.

Thanks for you help with this.

Andy