Advantage Security Inc. offers several solutions so that a protected premise is not inadvertently compromised as landline telephone service is discontinued in favor of Voice over IP (VoIP) phone service.
The problems result when VoIP is installed and the adapter is connected to a DSL or cable modem for Internet connectivity.
An incorrect installation often happens because digital phone service and dial-tone is back-fed to all phone-jack wiring on the premises. This incorrect installation procedure by the installing VoIP provider (in many cases your cable TV provider) defeats line seizure, which is required so the alarm-system panel cannot be interrupted by a local phone call during an alarm transmission.
If someone in the alarm business tells you that VoIP will operate reliability, even in the best of circumstances, you should view that as insight into their technical credibility. VoIp failures are frequent and with potentially catastrophic results!
VoIP Compression Affects Alarm Communication
When an alarm system communicates via a POTS line – or Plain Old Telephone System – a telephone call requires the same amount of bandwidth whether someone is talking or during a silent pause. VoIP protocol, on the other hand, compresses the signal to minimize the file size it needs to transmit. The conversion process is what often causes alarm-system glitches. To compound the problem, the compression ratio used by VoIP providers varies greatly. Even within the same providers network, the compression ratio may be based on current network capacity.
Taken directly from the Comcast website:
How does Comcast Digital Voice® service work with monitored home alarm systems?
Comcast Digital Voice works with most modern, home alarm systems that use tone dialing and standard data protocols. Comcast does not guarantee that the Comcast Digital Voice (including, without limitation, the feed between the alarm panel and the Comcast-supplied eMTA) will be uninterrupted or compatible with each and every make or model of home alarm systems.