I took a quick look at the user manual for the SBG6850 and it appears there is no QOS support for this model. Your only option would be to bridge the SBG6850 and attach/buy another router that supports QOS.

QOS is only needed if your 1 Mbs upload speed is being saturated somehow on your end and not coming from your ISP. I don't typically hear the term "warble" to describe this type of issue, but rather that they hear a "choppy" voice. Is there any other Internet usage going on during your calls? If so, try to stop or alleviate this usage and see if the issue goes away. QOS may be the answer if this is the case. Is the problem happening all the time or especially during the evening hours? If so, I would suspect an exhausted/saturated upload problem at your ISP. Try running http://www.pingtest.net shortly after these bad calls to see if you can confirm any packet loss, high pings or high jitter as the source of your problem. If so, you will have to complain to your ISP because having QOS on your end will do nothing to solve this issue.

Your going to have to experiment a little to get to the source of your problem.....

As a last resort, you may wish to request from Voipo support that they set your ATA to use a lower bandwidth codec other than 711u as the preferred codec.