VoIP is very sensitive to various network issues. You can have really good uptime, however your VoIP quality can still be affected by some other issue on your network.
In my experience, the biggest problem causing voice quality is the end user's network. Specifically, the problem is usually periods of packet loss or high latency. These problems are typically intermittent -- they come and go -- so your VoIP quality fluctuates.
Usually, the hardware (router, etc) isn't to blame although it's a good idea to power cycle to hardware just to be on the safe side.
Here is a good checklist of some things you can do to troubleshoot:
1. Power cycle your router, modem, etc.
2. Consider whether you are doing backups over your Internet connection, downloading or streaming music, or anything else that might use alot of bandwidth.
3. Turn off all computers and other devices other than your VoIP phone so you can isolate the issue.
4. Run VoIP Spear or Ping Plotter for a long period of time to see if this spots a problem with your Internet connection.