Cell Number
Cell Number
[10 boxes for 10 digits of a U.S. phone number. The first three are grouped by parentheses. A hyphen separates the second set of three and the last four.] [Label titled "Your seven random digits" pointing at seven empty boxes.] [Label titled "Where you lived in 2005" pointing at three empty boxes preceding the seven.] Structure of a US cell phone number.

This comic references the pattern for US telephone numbers, which are ten digits in length. Unlike in other countries, there is no quick way to determine whether that number is for a land-line or mobile customer. In either case, the first three digits are referred to as the " area code ", a term dating back to before the proliferation of mobile phones, when specific codes were assigned to geographic regions. The next three digits had some rules based on the telephone exchange but as Randall says, those and the final four numbers are essentially meaningless.
In the early days of the mobile era, the geography-based numbering still applied to new mobile lines, so mobile phones would have the same area code as owners' home numbers. Late in 2003, US telephone service providers were required to support "number portability", meaning that customers could theoretically take their mobile phone number with them to a new provider, even when moving to a distant new location. In the early days this wasn't always very easy to do, but became commonplace within a couple years. Since most users opt to keep their numbers constant whenever possible, numbers generally stopped changing after about 2005, instead of shifting when people moved like they were forced to in previous years. Therefore, examination of a given phone number will likely tell you where its owner was living at that time, since their number would not have changed after 2005 due to the portability law.
"+1" is the international call prefix for the North American Numbering Plan.
Google Voice is an alternate voice over IP service. Upon signing up, users can choose any available new 10-digit number without regard to geographic area. Among other things, this allows the earlier users to choose "cool numbers" if desired, such as ones that correspond to phonewords or have a pleasing pattern. In the past, this "vanity numbering" was typically only available to businesses via toll-free numbers . Some mobile service providers began allowing similar customization after the portability law, but often still restricted new numbers by area code, keeping the availability of "cool numbers" low until Google Voice launched.
[10 boxes for 10 digits of a U.S. phone number. The first three are grouped by parentheses. A hyphen separates the second set of three and the last four.] [Label titled "Your seven random digits" pointing at seven empty boxes.] [Label titled "Where you lived in 2005" pointing at three empty boxes preceding the seven.] Structure of a US cell phone number.
