If you took it upon yourself to install the iPhone 3.0 tethering hack you may have noticed that you have a lot less voicemail than normal -- as in zero.
If so, it isn't because people are just leaving you alone -- they aren't. What happened is that installing that .ipcc (carrier settings) file that enabled tethering also disabled your Visual Voicemail.
You can verify this by pressing 1 then Send in the Phone application. This should dump you into your regular old voicemail box (remember those?) and if you have messages waiting (I had 11) that means that your VVM is hosed.
But it's your lucky day, I'll save you the 70+ minutes that I spent being bounced around between AT&T and Apple technical support. Simply go to: Settings > General > Reset and tap Reset Network Settings. Your iPhone will reboot and all should be well with your VVM again.
Now stay away from those unauthorized tethering hacks - scofflaw!