Anonymous: Nov 19, 2012
"It is a fact that such a scam is possible, but it does not effect any mobile or residential phone users, it is possible only with certain types of vulnerable business phone systems that use PBX(private branch exchange) or PBAX(private automatic branch exchange) configurations. Such type of configurations are mostly used by businesses, hospitals, government and private organizations. In such configurations, pressing 9 is a standard signal to connect to an outside line through call forwarding, so when you dial #09 or #90, the scammer gains access to your line and can exploit your phone line in any unfair means.
Such business phone systems are advised to disallow call transfers to these numbers and must be trained to handle such scammers. Mentioned in the reference section are definite guidelines from FCC to avoid such scams.
Therefore, mobile phone users are NOT vulnerable to any such scams, but yes, one should always avoid call activity from strangers, as a safety measure."