Click to viewIf you have successfully unlocked your iPhone, you may have discovered already that the voicemail button doesn't do anything at all. It won't give you visual voicemail, but you can easily get the voicemail button to work using any carrier following these steps:
1. First, take note of what your voicemail number is with your current provider (for example, in Spain the Vodafone voicemail number is 177).
2. Click on the phone button in your iPhone.
3. Click on the keypad tab.
4. Tap the following code:
The xxx is the voicemail number, in our example it will read: *5005*86*177#
5. Tap call.
6. After a second, the code will have been set.
7. Tap now on the voicemail button and it will automatically call your usual voicemail service.
This tip comes from the iPhone Dev Team wiki (which can't be linked to by their own request.) You can support their efforts following the instructions here.