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. This gives an automated phone system time to answer the call and then after the pause has occurred, receive the extension from the caller.
Your humble iPhone supports this functionality, and it's very useful when needing to dial colleagues at the office rather than on their mobile phone. TheiPhoneWebsite.com has a handy tutorial that shows you how to save a "pause" followed by an extension using the iPhone Contacts app.
In order to input a pause in your contact's phone number, you need to open the contact for editing using either the Phone or Contacts app on your iPhone or iPod touch. Tap on the number you wish to modify, and when the dial pad appears, press the "+*#" button on the lower left. This will change the dial pad as seen above and allow you to insert a pause (shown as a comma) into your contact's phone number. Add the extension to the end of the number, after the comma, and hit Save.
Now when tapping on the contact's office number to initiate a call, you will not be required to dial the extension after the phone system picks up. You will automatically be transferred to cell phone dialing nirvana, or at least to the person you were trying to reach.