Indystar.com reports on the incompatibility between home alarm systems and VoIP services such as CallVantage and Vonage, noting that most home alarm systems do not support VoIP and will require you to maintain a traditional POTS line, or subscribe to an expensive cellular service so that the system can call the security company in case of an alarm.
There are multiple issue here, one is the way alarm systems can take over a phone line if necessary, cutting off any in-progress phone calls so that they can dial out to the service center. The second issue is backup power. Unless residences have backup power for home routers, ATA's and cable/DSL modems, a loss of power will mean that alarm systems can't phone home in case of emergency. Third, the tones that an alarm system sends over the phone line may not always be properly carried by VoIP. Finally, there may be other incompatabilities having to do with how systems are configured, and specific requirements of some older systems.
Then of course there is the issue of reliability of the VoIP service itself.
While some folks have had success getting their alarm system to work over VoIP, VoIP is generally not supported by alarm companies and users must be willing to accept the risk of using a non-supported solution (See this discussion at broadbandreports.com for examples of user attempts to use VoIP with thier home alarm systems)
The National Burglar & Fire Alarm Association (NBFAA) an industry trade group, has already issued a sample letter (pdf) for customers who are considering VoIP services, and they are urging members to develop plans to support residential VoIP services. It has also facilitated discussions between alarm system companies and VoIP service providers (See: Coming to Terms with What VoIP Means For The Alarm Industry - SecurityInfoWatch.com)
As I noted several months ago, the inability for VoIP services to support my home alarm system is the primary reason why I'm still relying on POTS for my home. VoIP just won't save me any money if I still need to maintain some sort of service for my alarm system.
In addition, I wonder how long it will be before an unsuspecting consumer who thinks they are just buying a "broadband phone service" without understanding the limitations of such a service, will suffer a loss and file a lawsuit against the VoIP vendor, much as we saw with regard to 911 service issues in the last few months.
Hopefully the E-911 issues have awakened VoIP vendors to the need to educate their customers about these sorts of things before the government decides to get itself involved.