This is an intermittent problem but a frustrating one:
We will be talking to someone on our Comcast Voice service, but suddenly we'll get dead air from the other side of the call. We can tell the other person that we can't hear them and will call back. Rebooting the modem doesn't seem to fix the problem.
If an employee doesn't respond to your message here . . .
Troubleshooting problems like this gets tricky. It would probably be best to contact them at the phone number on your bill or 1-800-Comcast, or use one of the options on https://www.xfinity.com/support/contact-us/. They usually can't fix problems like this remotely. If they can't, insist that they send a tech out to identify the cause and correct it.
If the tech finds bad coax, splitters, amplifiers, or connections in your home (even if Comcast originally supplied them) you'll probably have to pay for the visit unless you have their Service Protection Plan (https://www.xfinity.com/support/articles/service-protection-plan, closed to customers that don't already have it). If the trouble is due to a faulty Comcast phone modem or anything outside your home, you shouldn't be charged.