Not Legal Advice: The process of calling someone from a mobile phone and having another number show up in the caller ID is called Caller ID spoofing. This process is now illegal in the United States.
However, regardless of this law, there are still companies operating Spoof caller ID, including www.spoofcard.com/. The company claims that as long as you are not using their service in desecration of the law with the intent to "defraud, cause harm or wrongfully obtain anything of value”, then caller ID spoofing is still perfectly legal.
SpoofApp is now available for Android, iPhoneBlack, Berry, webOS, and even Windows Mobile directly from the developer's web page. You can check on how to install SpoofApp via Installer here.
In the news: Technology allows scammers to spoof Caller ID information quite easily and telephone companies are at a loss to figure out how to stop it. The Federal Trade Commission is at the vanguard of these efforts, which would help them fight telemarketing fraud and thievery. Find out more here.