If Steve Jobs says the iPhone comes with Visual Voicemail, the iPhone
really comes with Visual Voicemail. We simple users of the device don't get to disable it. That is unfortunate for those of us who travel internationally, because voicemail is really expensive when roaming abroad. The problem is that phone networks are also really dumb. All calls have to go through your home country, where they're forwarded to the roaming network, incurring international charges. Then, when the roaming network decides your call needs to go to voicemail, it sends the call back to you home network—incurring international charges a second time. You would normally call your voicemail box for the hat trick, and most of the time, the message is "Can you call me back?" or, after a long story, "I'll just e-mail you." (Can you tell that I'm not a big
fan of voicemail?)
With Visual Voicemail, the first two steps and the last one are the same, but the good part is that you normally don't have to call voicemail: it's delivered to your iPhone where you can admire it in full visual glory. It turns out that the iPhone downloads voicemail messages over its 3G (or 2G) data connection, but not over WiFi. So, when I was in Dublin (where I turned off 3G data roaming to avoid the insane data roaming fees), I was presented with a badge on my phone icon that told me I had a message. But because the iPhone couldn't, well, phone home, it couldn't download the message—or even tell me how many messages I had. The badge was just a sad, empty circle. Strange.
However, if I had investigated a bit more, I would have known that even though the iPhone GUI doesn't let you disable voicemail, you can do this using industry standard GSM codes. If you look here or here you can find long lists of these codes. Experimenting with these codes shouldn't be harmful, except for one thing: you may "unregister" your voicemail forwarding in a way that it's not possible to manually enable it again unless you know the number. (The iPhone will probably do this for you if you restore it, but I didn't test this.)
The first thing you want to do is discover the number of your voicemail box. You can do this by typing *#61# and pressing "call." The iPhone will now tell you if voicemail is enabled and the number of your voicemail box. Copy down this number and keep it in a safe place.
Now that you know your voicemail box number, you should be able to forward all calls to it using Settings - Phone - Call Forwarding. This is—among other things—useful when roaming internationally: because this type of forwarding isn't conditional, the incoming calls don't have to go through the network where you're roaming, saving you a lot of money. (This is all in theory, and your milage may vary.) However, this didn't work for me. In fact, I was unable to set up call forwarding at all, regardless of the number.
And now, finally, disabling voicemail on the iPhone: type #004# and call. Enabling it again is done with *004# and call. I made two contacts for these numbers and made them favorites, so I can just "call" "VM disable" and "VM enable" from my list of favorites. You can also forward calls to a different number, or selectively enable/disable diverts for no answer (61 rather than 004), not reachable (62), or busy (67), and there are some other tricks as well. See the lists linked above.
Post scriptum: These codes generally work on all GSM-type networks, but there are no guarantees. I asked Jacqui, our fearless editor, to try this on her AT&T iPhone in the US, and it didn't work for her. Deactivation seemed to work, but both the "activate all conditional diverts" (*004#) as well as the individual ones (*61#, *62#, and *67#) resulted in errors. Despite all this, her voicemail kept working. Apparently, AT&T (and other US carriers) don't let their users disable voicemail—the only way to get this done is to call customer service.
