Don't worry too much about this as it looks like some of your friends are trying to prank you. My nephew tried to prank me a while ago using such a phone service. Here is how it works: You go on the web and record the message you want to send to someone then you type the phone number of that person. Then you press send and the server calls and plays back the message to the pranked person. At the same time, it records your reaction so people can listen to it later on and laugh about it ...
That's not always funny to be honnest ... This kind of features of the WEB should be controlled a bit more.