Having friends with an office job in the typical corporate environment often times means having to dial a desk phone with an extension. As Lifehacker points out, this usually results in having to memorize a contact's direct extension for use after dialing the office's main line.
The difficulty comes from the fact that simply dialing someone's extension immediately after the main line number will not work. The internal phone system of the company you are calling will not recognize the extension has been dialed until there is a connection between the caller and the system.
To solve this problem, cell phones have functionality that allows users to save a "pause" in between a main line number and a contact's extension
