VOPI = Voice Over Private Internet. They are calling to get you to sign up for monthly services such as "Vonage", an internet based telephone. All calls are routed thru a Wifi connection that is password protected. While they are cheaper than any typical cellular carrier (like $10 per month), they are not 100% at all and subject to surprises for other services you may access without your knowledge. If you like Vonage and it works for you, great. But it is not practical for most people that need to know precisely what their bill will be each month.