Model numbers of the devices and how they are connected would help this thread a whole lot.

Is it just a Comcast modem, and a Grandstream Handy Tone 502 router/ATA? That is the simplest setup and should work. Are you a heavy computer user? Not sure how robust an HT502 is to support a lot of computer traffic.

You look to have registered here almost a year ago so I expect you've had a long good run before this issue. If the modem has not been power cycled in a long time, definitely do that.

VOIPo support (I especially like support@voipo.com) can do much more advanced troubleshooting than the user base here. Hope this gets resolved. Perhaps it is simply a network/Comcast issue that might pass soon?