Directly from Comcast.
""Comcast does not guarantee that XFINITY Voice service (including, without limitation, the feed between the alarm panel and the associated voice/data modem) will be uninterrupted or compatible with each and every make and model of home alarm system. If you have connected XFINITY Voice service to your alarm system and there is a power outage to Comcast’s network, your XFINITY Voice service will cease to operate during the power outage if you do not have a backup battery, thereby preventing inbound or outbound communications to or from your alarm system. Use of a backup battery may, but is not guaranteed, to provide limited additional service for your home alarm system during a power outage.
Unlimited Select and Local Select Customers (Virginia and West Virginia only):Service (including 911/emergency services) will not function during a power outage. A power outage will interrupt communication to and from non-voice communications equipment, including but not limited to, alarm and security systems or devices, medical monitoring devices, personal emergency alert devices, home detection devices and fax machines connected to Unlimited Select or Local Select.""