Now a hack has shown that Windows Phone already has all the right plumbing to make it work, and AT&T simply needs to allow Windows Phones to register for Voice Mail on their system.
For the hack to work the phone needs to be running Windows Phone Build 8107 which is likely the most complicated part of the hack (some instructions can be found here). One also need to borrow an Android LTE phone to register Visual Voice Mail on AT&Tâ€™s system and edit the APN settings on your Windows Phone, but people who tried it report in this XDA-Developers thread that it works pretty well.
The instructions are as follows:
1. Update your phone to 7740
2. Update to 8107
3. Insert sim card into an AT&T Android LTE phone
4. Go to att.com and switch plan to Data Pro 4g LTE plan. If the website does not work you may need to call customer support.
5. Setup VVM on the borrowed Android LTE phone (make sure your friend knows the VM password so they can re-add it when they put their sim card back in.)
6.Insert sim card back into Windows Phone
7. Call Voicemail
8. Wait for VVM to show up on phone (mine showed up after calling voicemail).
9. If sync error, edit the APN as follows
APN settings (delete the internet_line APN)
Create a new APN
APN Name: pta
Profile Category: internet (always on)
Auth type: None
Proxy address: proxy.mobile.att.net
Proxy port: 80
Before anyone goes flashing their phone with a 8107 ROM however there is a rumour that when 8107 does eventually come to AT&T Windows Phones, it will bring Visual Voice Mail support with it, and it may be safer to simply wait it out.
Read more in this XDA-Developers thread.