VoIP poor sound quality

We are using phone booth with t28 yea link phones. I have Comcast with 4 different yea link phones. All computer and phones are on same linkage srv4000 router. We have a clicking on the line and word clipping. I have taken the phone and plugged it directly right into the Comcast router and there was still a problem. I took the phone to my home with fios and did not experiience clicking and word clipping. I have heard about qos. Any suggestions.
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VOIP quality is based on the bandwitdh provided and routing. (Shortest route to the VOIP provider)

You could also enable QOS for Voice on the router, firewall or any other device you might have.

We know that the phones are not a problem since they work at home.

So it has to be the router settings or the bandwidth. If it is being shared, test the phones without anyone else using the line.
Time to either find a new provider or upgrade the internet service. What is the upload/download speeds? Do a speed test at speedtest.com to find out your actual speeds. Try to use a test server that is close to the sip providers location. If they have multiple proxy's try each one to see if there is any improvement.

Keep in mind that while you can set QoS flags, if the networks you traverse to get to your provider don't honor thode flags you have no control of the quality. We often find that providers usually have QoS in place only if they provide VoIP services of some kind.

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Word clipping will be caused by packet loss. The first thing to do would be to try the phone in the office where it is the only thing using the internet connection (by unplugging the other connections to the router) and see how that affects quality. If its much better then implementing QOS on the router may help as in this case the most likely cause would have been your internet connection maxing out its bandwidth.
If it doesnt help then it is most likely your internet service provider dropping packets because either there is a fault on their network or they network is too busy. Try running a ping test across the network (the windows program winmtr is very good for this) and raise a support ticket with your isp complaining about packet loss and attach the ping results. If they cant help then switch to a better isp.
Try testing it with one of the links below. On the first one try to chose a location close to your provider. This should help you find out why the quality is not there.

Voip Speed Test
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