Since you just picked up the CGN3 today, have a look at the following post which summarizes the present situation with the CGN3 (I'm assuming CGN3 here);
There are latency issues with the CGN3 and I'm wondering if that is coming into play here given your comment on the fact that it didn't happen when you were on a different router, 3825 by chance? I wonder if the only way to solve it, until the next firmware release is to run the CGN3 in Bridge mode and use your own router to do everything such as firewall, port forwarding, wired and wireless support etc, etc. If anyone is using a CGN3 in Bridge mode with a 3rd party modem behind it, and using Mumble, please chime in here and indicate if Mumble works, or if it still has issues.
Can you check your firmware version by logging into the CGN3? The latest firmware version is 4.2.4.3. If you have 4.1.4.1, it will take about 4 to 5 days for Rogers to push the next version out to your CGN3.