iPhone: If you already know the extension you’re going to dial, it can get really annoying to speed dial a number and then have to wait to enter an extension. Fortunately you can easily solve this problem with a comma.
You may not have noticed it, but when entering a phone number into a contact on your iPhone you can press the +*# button on the dial pad to get a couple of extra options. One of those options is “pause”, which inserts a comma into the phone number and serves as a friendly pause when dialing. This is very useful when dialing a phone number with an automated answering system. That pause will let the main number dial, wait a moment, and then dial the extension so you don’t have to. This is a great little trick to save you some time and effort when calling office directories and customer service lines.
How-To: Save Phone Extensions to iPhone in Contacts | The iPhone Website