On my router, a LinkSys WRT54GS, the admin app under "Applications & Gaming" -> "QoS" I am able to set device priority by MAC address. If I set the machine running the MJ to "High" or "Highest", would that be helpful? No real problems right now except for the random call with poor voice quality, but maybe I can get rid of even that. My router firmware version is 1.50.6.
That will give priority to your MJ PC over all other devices inside your home. Then, you can prioritize the MJ softphone to have higher priority within that PC. That's the limit of what you can do. If the congestion is outside your home you can't exercise any control over that.