After hacking the iPhone to enable Internet tethering without paying your mobile service provider, chance are the MMS (Multimedia Messaging Service) and/or VVM (Virtual Voicemail) functions on the iPhone, be it 1st Gen iPhone 2G, iPhone 3G or iPhone 3G S, may not working anymore.
The problem or error happens because when enabling Internet tethering by loading a new Mobileconfig configuration profile, a new .ipcc (carrier settings) file that enables the tethering, but disables the Visual Voicemail and/or MMS is been installed.
How to fix, restore and re-enable VVM and MMS in iPhone, and most importantly, keep the Internet tethering service intact? The solution is indeed very simple. Just go to Settings -> General -> Reset on iPhone, and tap Reset Network Settings. The iPhone will reboot. After the restart, MMS, VVM and tethering should be working fine again.
Alternatively, go to Settings -> General -> Network -> Cellular Data Network, and set the APN for Visual Voicemail to acds.voicemail. However, doing so will disable Tether and the option to tether will disappear.