The new iPhone 3.0 software comes with the ability to tether your phone to your laptop via Bluetooth or USB so you can use it to get on the internet, but for subscribers in some countries the feature comes disabled when you upgrade to the 3.0 software version. However, some people discovered a way to supposedly use Beta 4 of iTunes for Windows (specifically version 184.108.40.206) (I hear it works on any iTunes version on the Mac) and a special carrier update file ending in the .ipcc file extension to enable the tethering option on their iPhone 3G units now.
The problem that some people didn’t realize at first when they posted this update method, was that the .ipcc file also changes an option that disables Visual Voicemail and Missed Calls, at least in some cases. I don’t recall which of the many sites it was where I read this, but the solution is quite simple for anyone out there who might need it. From the Home screen, navigate to Settings->General->Network->Cellular Data Network. The section called Visual Voicemail has a field called “APN” and if it’s set to “wap.cingular” and you use the US iPhone carrier, that appears to be what’s causing problems. The “APN” field should read: “acds.voicemail” (without the quotes).
Note that this is for informational and troubleshooting purposes only and changing any of the settings or files here is not something I endorse. Tethering is a service that you may or may not be allowed to do and is between you and your service provider; please refer to their agreement terms for details and specifics. I didn’t come up with the solution above, I just happened to read it somewhere and I’ve been contacted by a couple of people who mentioned they had problems that were fixed when I pointed out the above change that I read about somewhere.