No need for going "postal" (pun intended) with a personal attack over my mention of POTS lines. I never said that a POTS line was the only option, nor the most reliable. I was also not advocating or taking a position on the value of alarm systems. If you don't need one, fine. The OP has one, and was asking about connectivity.
I was merely pointing out that, if one's goal is relatively reliable monitoring of a security system, then select a connectivity method that is likely to be RELATIVELY more reliable, compared to all choices. Your mileage may vary, depending on your locale and service providers. I just do not believe Google Chat/Google Voice being as reliable as one of the more established VoIP providers, or a cellular or broadband, or a POTS connection.
Since you and I both mentioned Nextalarm, go dig through their site, where they explain why conventional VoIP service is a poor choice. The issue is that the alarm panel generates analog tones (originally designed for POTS line specs), which the VoIP adapter digitizes, and sends over the (usually) public internet. Then, at the far-end telco CO, the digitized data is converted back into analog tones, and ends up at the monitoring center, with varying degrees of accuracy, depending on codec used, connection quality, etc. This is the same reason fax transmissions over VoIP are troublesome at times. Nextalarm's solution, ironically, is VoIP-based, but instead of going through transitions to the PSTN central office and back again, they use a custom-configured, dedicated Cisco/Linksys ATA which connects directly to their own digital back-end, like an extension on a IP PBX. That way, not only do you get better end-to-end signal quality, but they can also monitor the connection to the ATA and detect outages.
Now, certainly, someone is going to chime in and say that their VoIP service works fine with their alarm provider, and that's great for them. I'm just saying: decide which balance of cost and reliability is important to you.