This is a commercial product, not residential. I don't think Vonage provides any SLA to their customers, nor it is regulated to demand some minimal reliability standards. They are working in "best effort" mode. While it is good for day to day regular phone use, I think it is not suitable for real time alarm system.
I experienced last 15 years of 70 years of communism which should now be called "Occupy Russia". I hope to never see this in the United States.
For example, Verizon advertises three nines.