"I just got a call from this number. I ignored it, but i am worried that its one of the ones that can tap into your phone and use the money in your account, as when you pick up, they can hang up, and then they have full access to your account to use all your credit!!!"
Don't worry about this - its not possible with a normal home phone. It is possible if you have some types of PABX that allow incoming calls to create an outgoing call, the scam being they call you (local call) and then get your phone system to forward them to a more expensive destination (mobile, overseas, premium)
The reason for hang ups is normally a poorly built automatic dialler - to keep costs down the generate far more calls than they have operators, and when someone answers it will connect the call to a live operator. If it is badly set-up, there may well not be an operator free when you answer the call, so the call gets dropped.