Depends what you mean by fake.All sip devices are able to issue any desired callerid but legally numbers need to be used in accordance with regulations. It is possible to find out the originating device used for making the call.In the case of mobile phones new services like Riphub allow users to legally change the callerid to suit but again the call can be traced back to the underlying mobile device used if necessary.Changing numbers is very useful for all sorts of legitimate reasons and will become more prolific over the coming months.