[Diagram outlining the true structure of a US Cell Phone number. The 3-digit exchange number plus 4-digit subscriber number are your 7 random digits. The area code is where you lived in 2005.]
Structure of a US Cell Phone number.
There's also a +1 sometimes, which is there to keep everyone on their toes. In the future, people who got to pick cool numbers by signing up for Google Voice early will be revered as wizards.