call quality, QoS and VoIP

We are considering VoIP at one of our locations and the debate over VoIP is call quality.  We are considering Comcast and figure that we may experience issues with call quality using their Business package with VoIP.  Is there a way to have QoS and guarantee call quality on a cable service using VoIP?  Do we need to have T1 in order to have the best call quality?  Or if calls are important, should we just stick with the good ol fashioned PSTN/POTS network.

Your thoughts on this subject would be appreciated
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Bryant SchaperCommented:
You can improve call quality with QoS, but this is a bandwidth tool, so it is only useful when you saturate your bandwidth, if you have 100megs and only use 10, then no QoS really happens, but the carrier network may support it.

Now is Comcast over the internet on a none Comcast circuit?  Then no QoS anyways.  It is certainly possible, we use SIP from our MPLS provider, delivered to a cisco Call Manager at our data center then we have 20 remote offices that connect to the data center for calls.  You may want to look into your codec options and lowering the call quality to improve bandwidth usage, such as g729 over G711.  Some people don't even like G711, it is too clear, none of the background noise like an analog line.

Now faxing over sip is a challenge so pay attention to that as well.

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1) QoS works only if you have MPLS link or P2P link between locations
2) QoS dont work on internet
3) You need to have QoS agreement with ISP.

what you exactly mean by cable service ?
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