Security with regard to what? Someone listening in on your calls? It working when power is out? 911? Reliability? If it's someone listening in, I've never seen it discussed. The traffic goes over the internet, unencrypted, AFAIK, with all the VOIP services. So, it would seem about as secure as any similar internet traffic. If someone wants to listen in, is smart enough, determined enough, they can, just like with a phone call on the old copper system. It's probably more secure than that, because with the old system you could tap in with stuff not much better than an ordinary phone by getting to the wires in the building, going to the street, etc.