On dialling back: don't forget there is a scam base on the way a telephone line disconnects that means it is not sufficient for YOU to put the phone down if they rang you, THEY need to do so. So don't call back immediately. Instead call something or someone else first to make sure you are really disconnected from them. Secondly, don't call a number they give you - how do you that is genuine? If it is your isp for example, look up their number yourself.