In what will likely cause a bevy of discussion at the next VoIP conference, hackers have revealed some vulnerabilities within VoIP networks that make spoofing phone numbers even easier, and provide an ability to see the numbers that are call blocked in this item posted in The Register.
While they have lowered the cost of doing this to "free" it is nothing new to those of us who have had advanced telephony services, or 800 numbers.
For starters, my Webley service has provided me phone numbers that were blocked for years (you should have heard the reaction from a San Diego County Sheriff who called me on his "blocked" cell phone when I called that number back, not the number he left after reporting an incident).
As far as spoofing outgoing numbers, as the register points out, anyone with a big enough PBX has been able to do that for years. Sales organizations regularly do that to prevent the parties receiving their call to know who is calling.
I actually wish I could change the outgoing number on my VoIP lines to the main, published number I use, and then, using advanced IP based features manage what calls come from whom to me, when, just like I can do with Webley, minus the ability to change the outgoing number. Of course on Caller ID Webley shows up as either an 847 or toll free number, masking my location and providing some degree of discretion.