If you experience any of the following, use this document to help troubleshoot the issue:

Unable to access the Internet on my computer using iPhone
Unable to enable Internet tethering on iPhone
iPhone displays "Internet Tethering" whenever I sync my iPhone to my computer or when I connect to my computer for charging
Internet tethering speed is not as fast as expected
Internet tethering connection drops when placing or receiving a phone call
An error message appears after pairing iPhone with the computer using Bluetooth

This article explains how to replace your current iPhone with a newly purchased iPhone using the same carrier. If you follow these steps to back up your previous iPhone first, and then restore the backup to your newly purchased iPhone, your saved SMS messages, email accounts, photos, notes, and other personal settings will be present on your new iPhone.

Use this article to find links to wireless-carrier support and basic information about the services currently supported. Click the region in which you live to locate your carrier’s information.

When using a POP email account with multiple devices, email messages may only appear on the first device that receives them. This occurs when an email client is configured to automatically remove email from the server once an email client has received it. You can configure your email client (or clients) either to keep a copy of the email on the server, remove it after a certain number of days have passed, or keep it until you move it from your Inbox.

This article explains the basics of Apple Push Notification Service and where to get additional information.