This is actually quite a simple fix and it doesn't require you to download a .cfg file...
If you no longer receive voice mails it's because you're running a profile that's not up to date. The best way to enable tethering through your iPhone is through Cydia and not a downloadable profile to your iPhone.
Anyways, if you do not receive voice mails anymore, here is the simple fix.
Settings > General > Reset > Reset Network Settings
And back to the iPhone tethering... There is a program on Cydia in which you can unlock a source and find a bunch of Cydia applications "jailbroken" in a sense. They're paid applications through Cydia that are free.
And that's the easiest way to enable tethering through your 3G or 3GS.
To find that and other great free Cydia applications like "Quick Reply SMS" you just have to:
Cydia > Manage > Sources > Edit > Add > http://www.sinfuliphonerepo.com/
You can enable tethering there through that source and find a bunch of awesome jailbroken applications through Cydia as well.
*NOTE* before you do enable your tethering through Cydia, make sure to delete your old profile that enabled your tethering.